David Mendoza first realized something was off in the summer of 2006, when he was rear-ended by a US mail truck.

He had been driving through Portland, Oregon, headed south toward Bend, where his construction company—called ETA because of Mendoza's reputation for being late—was restoring the historic Liberty movie palace, which Mendoza had purchased a few years earlier. "My back bumper flew off into the other lane," Mendoza says. "I picked up the bumper and saw this device with batteries and a long wire. I took it to the Chevy dealership in Bend, and the guy said, 'Dude, this is not good. That looks like a GPS device.'"

Mendoza showed the device to his attorney, who suggested he start checking other cars affiliated with his businesses. In addition to ETA Construction, his businesses included a Seattle pizzeria called Pazzo's and an international marijuana-smuggling consortium. Mendoza found GPS devices on three company trucks and his girlfriend's car. "They were tracking us," Mendoza says. "The heat was on."

In reality, the heat had been on for at least three years. In April of 2003, according to court documents later submitted by the office of US Attorney Jenny Durkan, customs seized 400 pounds of Mendoza's marijuana hidden in a load of lumber on a semitrailer crossing the Canadian border.

And almost a year prior to the mail-truck incident, an unusual couple had moved in across the street from Mendoza's house in Bellevue. Every day, "like clockwork," Mendoza remembers, a man and a woman would take 12-hour shifts at the house. The woman came home at 6 a.m. and stayed until 6 p.m., when the man came home and stayed until 6 a.m. They were rarely there at the same time, didn't seem to have friends, and rarely deviated from their routine. The owner of the house they were renting, a friend of Mendoza's, hinted that they weren't typical tenants.

One night that summer, after the couple moved in, Mendoza left Pazzo's pizzeria—which he still owns on Eastlake—and went to a sushi place down the street. His suspicious neighbors from across the street were there, eating with two other strangers, and they all seemed startled when he walked through the door. "The shock and look on their faces when I walked in was priceless," Mendoza says. They were all FBI agents, he now suspects—two monitoring his house and two monitoring his business—who'd gotten together for a dinner break.

Around that time, an ex-girlfriend told Mendoza she'd been called before a grand jury. A few weeks later, an old acquaintance showed up unannounced at the apartment Mendoza keeps above Pazzo's, saying he "wanted to talk."

Mendoza assumed the friend, whom he hadn't seen in years, was wearing a wire. "How else did this guy even know about the apartment?" he says. "I told him to come back tomorrow—but I already knew I was leaving the next day." He says he'd already pulled the plug on his marijuana business, after a scare earlier in the year when he saw one of his cross-border smuggling helicopters on an ABC News program. But, he says, "it takes five years for an indictment to go stale." So he was headed to Mexico and then to Spain, maybe for good.

Mendoza was born in California but has spent his life shuttling between the United States and Basque Country, where his mother was born and where most of his family—who for a time helped raise him after his American father died—still lives. He'd bought some property just outside the town of Guernica years earlier and had recently begun building a house for his girlfriend (now his wife) and their children. By October of 2007, a little more than a year after the US mail truck knocked the bumper off his car, Mendoza began the process of applying for Spanish citizenship. If he became a dual citizen, prying him out of Europe to stand trial in the US would be more difficult—it would require a request from the US to extradite him. Even if Spain agreed to extradite, and even if he were convicted, he had a fighting chance of serving his sentence in Spain.

Mendoza says he wanted to do his time in a Spanish prison for the sake of his immediate family. One of his two young children has Down syndrome, and the Spanish penal system puts much more emphasis on keeping prisoners in touch with their families, sometimes allowing them to spend days and nights together in special "family units" designed for that purpose. (Historically, the American prison system has emphasized solitude as a form of punishment—originally advocated by English social thinkers of the 1700s as an opportunity to pray and reflect on one's moral shortcomings—but contemporary European prisons increasingly focus on socialization and rehabilitation, including regular contact with family and the outside world.) Mendoza figured that if he could serve his time in Spain, he would relocate his wife and children to live with his extended family, allowing them to visit him.

And the legal precedent looked promising. There is a long-standing tradition in international law, practiced by European countries and endorsed by the United Nations and the European Court of Human Rights, to support the repatriation of prisoners: Country X agrees to extradite one of its citizens to stand trial in country Y on the condition that the convicted citizen will be returned to country X to serve his sentence. "Repatriation, transferring a prisoner to serve a sentence back home, is universally recognized as a good thing for rehabilitation," says Ben Cooper, a UK attorney who represents Mendoza and specializes in thorny extradition cases, including that of Gary McKinnon, a young UK man with Asperger's syndrome who was accused of hacking into NASA and US military computers (a US prosecutor called it "the biggest military hack of all time") to find evidence of UFOs and secret technologies. "The Dutch do not even extradite unless the US agrees to repatriate."

Wesam al-Delaema, for example, is an Iraqi Dutch national who was born in Fallujah, became a Dutch citizen in 2001, returned to Iraq several times (during one trip, he made a film titled Fighters of Fallujah), and was eventually arrested in the Dutch city of Amersfoort. The US asked to extradite him so he could stand trial for conspiring to kill American soldiers with roadside bombs. Dutch courts agreed, on the condition that he be repatriated to the Netherlands to serve his time. Al-Delaema pleaded guilty, was sentenced to 25 years, and returned to the Netherlands where a Rotterdam court released him after serving six years.

And in 2009, a Barcelona man named Eduardo Sanchez-Moragues was arrested in Spain and extradited to the United States to face charges of sex tourism, including "illicit sexual conduct with a minor" with the help of the girl's father. After Sanchez-Moragues pleaded guilty in a federal court in Pennsylvania, special agent John P. Kelleghan of Immigration and Customs Enforcement said: "Traveling across the ocean to sexually exploit a child whose father was complicit has to be one of the most depraved and heinous crimes a human being could conceive." In its sentencing memorandum, the US Attorney's Office wrote: "The United States will affirmatively recommend" that Sanchez-Moragues be returned to Spain to serve his time.

If these kinds of criminals—Iraqi insurgents, child predators—were able to return to Europe to serve their sentences, why not a marijuana smuggler?

David Mendoza had been dealing drugs for decades, but he's no hard-luck case. Raised in the gated community of Broadmoor (85 acres of luxury homes surrounded on three sides by a 115-acre golf course), Mendoza became a class president at Garfield High School and graduated from the University of Washington in 1989. By his own account, he began smuggling marijuana around the time he finished college, driving up to Canada and returning with a few pounds at a time, hidden in a spare tire. "We'd make around $1,000 off each pound," he says. "We'd get six or seven pounds in, it would take a month or two to get rid of, and there were three of us involved. We realized, 'Hey, there's a lot more to this.'" He and his friends graduated to walking dozens of pounds across the border in rucksacks, taking three-hour hikes along logging roads in the middle of the night.

They also had some misadventures in the cocaine trade. Mendoza was the conduit between friends in Spain who could sell the stuff in bars and friends in America with delusions of grandeur. They aspired to become jet-setting cocaine smugglers, but, according to a 1993 sentencing memorandum filed by Mendoza’s defense attorney Kate Pflaumer (who would later become a US Attorney), Mendoza was a minor player in a group that was "largely unsuccessful, both in acquiring a kilo of cocaine and in exporting it."

According to court documents, they'd gone to Florida in May of 1989 to buy a kilo, but their supplier took the money and disappeared. Six months later, Mendoza bought a kilo in Las Vegas but was busted by airport security on his way back to Seattle. Their third try was slightly more successful. While Mendoza was in Spain, the others went back to Florida, bought a kilo, and got it back to Seattle. According to the US Attorney's Office, one of the friends—Chuck O'Connor—cut the kilo so he could take 200 grams to Spain to pass along to Mendoza. But O'Connor was caught snorting some in an airport bathroom and, afraid of being busted, "hid the cocaine in the airport furniture." Mendoza went to the airport separately to fish it out. This, as far the court knows, was their one successful international score. A few months later, O'Connor went to North Carolina to buy another kilo, but Pflaumer's sentencing memorandum says he'd been sleeping with the girlfriend of the dealer, who got his revenge by selling O'Connor a kilo of useless powder. They gave up after that but were eventually arrested. Mendoza was sentenced to 70 months, but was back out by 1997.

Meanwhile, their pot business was going well. They had been importing hashish from Spain and continued to smuggle from Canada. When they got tired of hiking it across the border in rucksacks, they began driving it across in semitrailers. Mendoza says his construction business was taking off and he'd order shipments of lumber from Canada, with a hollowed-out gap in the boards to be filled with hundreds of pounds of marijuana. "Weed has a similar density to wood," Mendoza says, so it could pass undetected through cargo-scanning machines at the border. "But one of the trucks got busted," he says, "when they didn't fill up the hollow part all the way. When it went through the density X-ray, they saw six inches of empty."

"So," Mendoza says, "we decided to use a helicopter!"

He and his associates arranged drops of hockey bags full of pot into the Okanogan National Forest, up to 500 pounds at a time according to court documents, but got a jolt in 2006 when they saw one of their regular helicopters on ABC's nightly news. "The pilot and the pickup guy decided they'd do backdoor deals," Mendoza says, "and they started working with another group who got in trouble—I found out by seeing it on Peter Jennings." (For the record, Peter Jennings died in 2005. At the time, Charles Gibson was the host of ABC's World News Tonight.) "I was like, 'Oh shit, that's our fucking helicopter!' We put two and two together, and the whole thing dissipated."

But it was too late. According to a 2009 sentencing memorandum filed by US Attorney Jenny Durkan, federal agents had been watching Mendoza since at least 2003—including one summer day in 2005 when agents found 500 pounds of marijuana packaged in nine hockey bags and hidden in a wooded ravine. They seized the pot then sat back and watched as Mendoza and two associates, one of them a pal from the old cocaine misadventures, tromped around the woods trying to find it.

In the summer of 2006, Mendoza found the GPS device after his collision with the mail truck. In December 2006, a grand jury filed a sealed indictment accusing Mendoza of conspiring to import more than 1,000 kilos of marijuana into the United States and several counts of "possession of marijuana on board an arriving aircraft." By early 2007, Mendoza was living in Spain. He applied for citizenship and became a Spanish national in May of 2008. On June 5, 2008, Mendoza was arrested by Spanish police at the request of US authorities and immediately began fighting his extradition.

But on August 1, 2008, the Spanish National Court issued a ruling that all parties, it seemed, could live with. It agreed to hand Mendoza over to the United States with four strict conditions: (1) no death sentence (because there is no death penalty in Spain), (2) no life sentence (ditto), (3) one of the charges, for "currency structuring," must be dropped (because there is no equivalent in Spanish law), and (4) "the sentence imposed in the United States can be served in Spain." The court was explicit about this, adding: "In this case, the American authorities have to agree as a condition of extradition."

Nobody knew it at the time, but this was a pivotal moment. The ramifications are still being felt today—not just for Mendoza, but also for accused criminals in Europe (some wanted for multimillion- and multibillion-dollar financial crimes) who are currently fighting extradition to the US, in part because of what happened next.

The US murmured some quiet doubts about the Spanish National Court's insistence that Mendoza be returned to Spain. For example: After he'd left the country, the US government had begun a series of forfeiture proceedings, taking properties they said were involved in Mendoza's drug business. ("That was a tactical move they did on me," he says. "They'd seize it and knew either I had to come back to the US to challenge them"—in which case, he'd be arrested—"or, in six months, I would forfeit it to them.") During one of those forfeiture proceedings, two months after the Spanish National Court insisted that Mendoza be able to serve his time in Spain, Assistant US Attorney Richard Cohen claimed that the Spanish court "is unlikely to uphold this condition." And in January 2009, the US embassy sent a carefully worded note to the Spanish equivalent of the State Department, saying: "The Western District of Washington at Seattle... does not object to Mendoza making an application to serve his sentence in Spain" (emphasis mine).

But the Spanish National Court was adamant, reiterating on several occasions that Mendoza's return to Spain to serve any US-imposed sentence was legal, consistent with their extradition treaties, and mandatory. "We are not dealing with an impossible condition," the court wrote in November 2008, "but one that is perfectly feasible through the Agreements of the European Council for the transfer of convicted persons, which have been signed and ratified by the United States and by Spain... In this case, acceptance of such a condition would not be optional."

In other words, take it or leave it.

The US took it. One rainy April morning in 2009, a small crowd of officials from Spain and the US met at the Madrid–Barajas Airport to transfer David Mendoza from Spanish custody to US custody. The Spanish police brought two actas de entrega ("deeds of surrender") for everyone to sign. The first acta referenced the four conditions of his extradition and was signed by three people: a representative of the Spanish police, a representative from the US embassy named Kimberly Wise, and the prisoner. To make sure the US law-enforcement officials were also clear about the terms, a second acta de entrega, also referencing the conditions, was signed by Spanish police and a US marshal. Then Mendoza and the marshals boarded Delta flight 127 and flew across the Atlantic.

Mendoza never returned to Spain.

After arriving in the United States, David Mendoza pleaded guilty and was sentenced to 14 years in prison.

The terms of the plea agreement, negotiated between prosecution and defense attorneys, included the forfeiture of four properties worth millions of dollars and the government's promise that it would not use evidence it had gathered from its investigation to charge his wife, Elizabeth, with any crimes. Mendoza also claims he agreed to forfeit some property that wasn't associated with the marijuana scheme to the US government after Judge Ricardo S. Martinez, who'd been presiding over the plea bargain, told him his transfer to Spain depended on it.

During the bargaining, each side thought the other was trying to pull a fast one. The US Attorneys thought they hadn't gotten enough out of Mendoza. "I do remember noticing that Mr. Mendoza appeared very pleased & chatty at the end of the settlement conference," Assistant US Attorney Susan Roe wrote to one of Mendoza's lawyers in an e-mail after the negotiations, "and I wondered just how many properties we had left in his possession."

Meanwhile, Mendoza had the sinking feeling that the US—despite the insistence of the Spanish court and the contract the embassy had signed on that rainy morning in the Madrid airport—had no intention of returning him to Europe. He first realized what was happening when the US Attorneys pressed him to inform on other drug runners. "I said, 'I won't cooperate, sentence me and send me back to Spain,'" Mendoza remembers. "That's when I began to sense something was wrong. Roe [the assistant US Attorney] began to smirk." Later, in its sentencing memorandum, the US Attorney’s Office wrote that the matter of where Mendoza would serve his time was “one that the State Departments of the two countries must resolve, and that the United States Attorney’s Office has no standing in [the] dialogue.”

"Mendoza pleaded guilty essentially because he knew he'd go back to Spain," Alexey Tarasov, another attorney representing Mendoza, says. "He relinquished a significant amount of real estate that was not scheduled for forfeiture... but decided to forego that process, plead guilty, and give up all of his assets in exchange for going back to Spain."

Five years later, Mendoza is still sitting in a prison in Fort Dix, New Jersey. The Department of Justice has denied all his requests to be transferred back to Spain, writing, in its most recent refusal, "There is no administrative appeal from this decision."

Mendoza's case, and the apparent US indifference to the requests from Spanish courts, have become minor news in Spain. A few articles have been written, there has been some parliamentary discussion of the situation, and influential judge Javier Goméz Bermúdez sent a notice to the executive branch of the Spanish government warning that "you may find future extraditions from Spain to the United States of America compromised" unless the situation is resolved.

But the US hasn't budged. "The extradition treaty between the US and Spain does not allow Spain to demand or decide where a defendant serves his imprisonment," Emily Langlie, a spokesperson for US Attorney Jenny Durkan, wrote in an e-mail. She added that the Department of Justice "discourages promises or recommendations by US Attorney's offices as part of plea discussions."

Both of those claims run counter to what's happened in other cases: Wesam al-Delaema was returned to the Netherlands because a European court demanded it. Eduardo Sanchez-Moragues was returned to Spain because of a recommendation by a US Attorney's office. And the Spanish National Court reiterated, over and over again, that Mendoza's return was consistent with its treaties.

"America is on the international stage before a sovereign court making promises in bad faith," says UK attorney Ben Cooper. "They're undermining their own credibility in a dangerous way."

Mendoza's case, it seems, has come down to a difference of opinion.

But the way the Department of Justice has treated Mendoza may be coming back to haunt it—neither Mendoza nor Spain can force the US to return him, but the whole story is now a cautionary tale to European governments considering other extradition requests from the US.

"It will make other countries less likely to extradite—that's just common sense," says James Frush, an attorney who has cited Mendoza's case to defend his own clients against extradition. Frush represents Michael Mastro, the former Seattle real-estate magnate who is wanted by the US for multimillion-dollar bankruptcy fraud. In 2011, Mastro and his wife moved to the French Alps and, a year later, were arrested by French police at the request of the United States. In the ensuing extradition battle, Frush says, French courts asked for guarantees from the US, including a promise that if the Mastros were found guilty, they would not be sent to prison—Michael Mastro is in his late 80s, and the court expressed concerns that the US prison system might not provide adequate health care. The US countered that it would provide adequate health care, but Frush says he pointed to the Mendoza case as evidence that the US could not be trusted. In June of 2013, a French appeals court decided to deny the extradition request. US observers were confused. "It's baffling to us here in the US why the French are letting on-the-lam Seattle developer Michael Mastro get away with it," columnist Danny Westneat wrote in the Seattle Times.

But Frush, like Cooper, thinks the answer isn't baffling at all—the Mendoza case has damaged US credibility. "This can ultimately harm US interests," Frush says. "As a former federal prosecutor, it really bothers me because it undermines the legitimacy of the process... If defense counsel can show that US promises aren't good, it has an effect."

The Mastros aren't the only people wanted for financial crimes that the US is having trouble prying out of Europe. Javier Martin-Artajo, one of the JP Morgan traders involved in the "London Whale" trading scandal (which resulted in $6.2 billion in losses for the bank, greater than the GDP of many small countries), is in Spain, fighting extradition to the US, where he is wanted for fraud. His attorneys did not return requests for comment, but Cooper says Mendoza's case has been part of their legal strategy. Russian citizen Dmitry Belorossov is also in Spain and also wanted by the US for fraud. Tarasov says Belorossov's legal team had contacted him and is using Mendoza's case as a way to deflect the extradition request.

Spanish attorney Carlos Gómez-Jara, who teaches law at the University of Madrid, says it's difficult to tell how far-reaching the consequences of Mendoza's situation will be, but that "it is definitely jeopardizing other cases." In one session of the Spanish National Court, he says, the judges agreed that all future extradition cases involving the United States would need additional and very specific guarantees. "They said the United States was like Russia," Gómez-Jara says. "Countries where you cannot trust what they're going to do."

Mendoza and his attorneys are trying one last effort to get him back to Spain. A few weeks ago, they filed a lawsuit against Attorney General Eric Holder for breach of contract, arguing that those deeds of surrender signed at the Madrid airport were a binding three-way contract between the United States, the Kingdom of Spain, and David Mendoza, a citizen of both. They're insisting on a jury trial, hoping a group of Mendoza's peers will undo the US government's decision to keep him in the United States. It's a risky and strange lawsuit, and Tarasov has no idea whether the courts will throw it out. "Lawsuits such as this are very, very infrequent," he says. "We are on the cutting edge of these kinds of legal disputes in the United States."

Whatever happens, Cooper warns that the United States should do something to resolve the situation. "They have a big responsibility to get this case right for everybody else," he says. "If they're interested in the return of truly dangerous criminals, they've got to play straight."

This article has been updated since its original publication.