County law enforcement cracked some heads this past weekend—or at least a bike helmet. King County detectives took two bruised Seattle bicycle riders into custody Friday, June 30, after a confusing tussle between Critical Mass bicyclists who blocked a Belltown intersection and two undercover King County cops who were startled when one of the bikers ran a red light and commanded them to stop their car.

To the bike riders who saw what happened, the arrests looked like the latest episode of police brutality in Seattle, coming right on the heels of last week's shooting involving an off-duty cop in Post Alley. However, David Speikers, attorney for accused 30-year-old cyclist Zack Treisman, who is under investigation for the felony charge of assaulting a police officer, is not describing the event as police brutality, but instead as simply "road rage" by the officer.

Speikers says Treisman intervened in what he thought was a fight between his cyclist friend and a gangster. "In this state it is legal to use reasonable force to help someone you think is in physical danger." It turned out the gangster was actually an undercover cop. "They started a fight as citizens and ended it as law enforcement," says Speikers, who says the detectives did not identify themselves before attacking the biker "with absolutely no provocation."

The whole incident got started when Jace Brien, 18, rounded a corner at the intersection of Western Avenue and Vine Street after biking up a long hill with several hundred other Critical Mass bikers. Seattle Critical Mass is a loosely organized ride across the city that takes place every month as a "share the road" demonstration by cyclists. They're usually a ragtag group of bike messengers, students, anarchists, and families riding to remind drivers, via peaceful civil disobedience, to respect bike riders' rights on the road. Friday's ride was the largest in months, with over 200 bicyclists filling the streets.

Riding up the hill on the approach to the intersection, however, the bikers got spread out. Roughly half of the cyclists had already passed through the intersection, blocking traffic as they rode through a red light. Critical Mass riders often encounter impatient motorists who try to drive through the demonstration and have a method called "corking" to keep drivers at bay as a herd of bikers rides through the light. So, Brien rode a little ahead of a clump of bikers and took up position in the Western and Vine intersection. He halted in the middle of the crosswalk, waving his arms at the cars and telling them to stop as bikers started whizzing through the intersection behind him.

The first vehicle in line at the light was a boxy brown van with civilian plates that started edging forward toward the bikers. Brien dropped his bike and waved his arms vigorously at the van. According to bicyclists, the men in the van yelled and made threatening gestures. Then, one of them stepped out onto the street.

"He looked like a Seahawks fan with a long, baggy shirt, baggy shorts, and sunglasses. A big beefy guy," says witness and Critical Mass rider Graylan Vincent who submitted a sworn testimony to Speikers. Brien, more on the string-bean side, claims he was afraid the guy was going to beat him up, so he immediately jumped on his bike and rode away. Another guy—equally bulky and baggy with a shaved head—got out of the van, chased after Brien and tackled him to the ground.

Vincent remembers the guy didn't say anything, there were no gestures or verbal warnings. "That was scary. The driver made a beeline toward [Brien] and grabbed him and threw him to the ground. He had this look in his eye like his blood was boiling." Several Critical Mass riders say they saw the man hitting Brien's head into the pavement and kneeing him in the back. A crowd of bikers formed, yelling at the beefy guy and snapping pictures with cell phones.

Treisman rode into the intersection and saw his friend on the ground with the "Seahawks fan" on top of him. Treisman dropped his bike and leapt onto the attacker, grabbing his neck in a chokehold.

That's when, witnesses say, the man yelled, "Stop! You're under fucking arrest! I'm a fucking cop!" Treisman and Brien didn't believe him until he pulled out a badge and handcuffs. "There's no way a reasonable person would have thought they were police," says Charles Redell, a Critical Mass rider who witnessed the event. "If they were police, they would have fought a traffic infraction with a ticket... not unprovoked violence."

The police were undercover detectives from the King County Sheriff's office. Sheriff's department spokesman Rodney Chinnick says that contrary to the witness reports, the officers repeatedly identified themselves. First, the officers sounded an air horn and siren noise while the first pack of bikers rode through the intersection. When Brien arrived a few minutes later and blocked their van, the officers say they sounded the air horn and siren twice.

After Brien dropped his bike and "challenged" them, Chinnick says, an officer stepped out, held up a badge and said, "You're under arrest." Brien fled and the second officer jumped out of the car and flashed a badge before tackling him. The officers also say they heard someone in the crowd of bikers yell, "What's the charge?" so someone, they argue, knew they were police. A possible explanation for the discrepancy between police and witness accounts is that the police wore their badges on chains around their necks. Since the cops were dressed in street clothes, witnesses could have easily mistaken the badges for hiphop medallions.

The King County Sheriff's office has jurisdiction over the investigation, so even though the altercation occurred within Seattle, the Seattle Police Department was not involved and is not able to comment.

The officer who Treisman grabbed in a chokehold (King County Sheriff's Department has not released the officers' names) was treated at the hospital for injuries and Treisman himself sustained bruising, contusions, and neck strain, according to his lawyer. Brien says he had only a couple scrapes, but his helmet was cracked.

Police found a few beers in Brien's backpack and released him Friday afternoon after charging him with possession of alcohol by a minor and disorderly conduct. Treisman was held overnight in jail, released on $3,000 bail Saturday evening, and is awaiting a hearing on Thursday to determine whether he will be charged with felony assault of an officer.

Treisman's lawyer, Speikers, hopes he won't even be charged. He has statements from 15 witnesses who–after getting the word on a biker message board—dropped sworn statements off at Wright Brothers Cycle Works in Fremont. None of the statements mention a horn sounding and none of those witness say they saw a policeman's badge until the two bikers were already on the ground. "Mr. Treisman didn't have any intent to assault a police officer... all he was trying to do was break up a fight," says Speikers

Speikers also emphasizes that the police response—tackling, kneeing, head slamming—was way out of proportion to the infraction offense of blocking an intersection. Speikers's defense is built on Treisman's legal right to protect Brien from what he thought was civilian assault.

Two weeks from now, Treisman is supposed to present his University of Washington graduate thesis in mathematics. Overnight in prison, he played chess with other inmates using ripped up scraps of paper.

Eli Sanders contributed to this story.