This has made the rounds on some news sites already, but in case you haven't seen it—based on the most recent data from the Bureau of Justice Statistics, Ben Casselman at FiveThirtyEight calculated that US Attorneys prosecuted 162,351 federal cases between Oct 2009 and Sept 2010.

That's 193,021 offenses investigated by the feds minus the 30,670 cases in which US Attorneys declined to prosecute. What were some of those reasons?

Table 2.3 in the BJS data tells us that 324 of those 30,670 cases wound up in pretrial diversion, 283 had witness problems, 3,679 were passed up because some other agency asked the US Attorneys to not prosecute (I'd like to know more about that figure), and so on.

And how many times did prosecutors decline to pursue a case because a grand jury refused to indict—as happened in Ferguson? Eleven times. Out of 193,000 investigations, 162,000 of which led to prosecutions, only 11 ended with a grand jury refusing to indict.

Granted, that's federal data and the Ferguson grand jury was a local proceeding (and we don't have comparable data for those)—but it indicates how rarely grand juries fail to give prosecutors the indictments they want.

As regular Slog readers know, prosecutors hold almost all the cards and all the power in a grand jury. There are no judges and no defense attorneys—just the prosecutor, the witnesses who have been subpoenaed (who can wind up behind bars if they refuse to answer questions), and the jurors. Hence the famous saying by New York State Judge Sol Wachtler that a grand jury could "indict a ham sandwich."

A prosecutor has to be almost—or actually—trying to come out of a grand jury proceeding without an indictment. Which is why it only happened 11 times in one year at the federal level, according to the most recent BJS statistics.

As a 2002 article in the Harvard Journal of Law and Policy put it: "Operating outside the public eye, the grand jury is the most powerful investigative agency in the federal criminal justice system." Unlike the FBI or other law-enforcement organizations, a grand jury investigation can jail witnesses who refuse to talk and subpoena documents and other evidence without a search warrant. No search warrant means no probable cause. Add in grand jury secrecy (at the federal level—local statutes vary) and you've got an extremely powerful investigative tool operating without much public oversight.

So what happened in Ferguson? Back to FiveThirtyEight:

“If the prosecutor wants an indictment and doesn’t get one, something has gone horribly wrong,” said Andrew D. Leipold, a University of Illinois law professor who has written critically about grand juries. “It just doesn’t happen.”

Cases involving police shootings, however, appear to be an exception. As my colleague Reuben Fischer-Baum has written, we don’t have good data on officer-involved killings. But newspaper accounts suggest, grand juries frequently decline to indict law-enforcement officials. A recent Houston Chronicle investigation found that “police have been nearly immune from criminal charges in shootings” in Houston and other large cities in recent years. In Harris County, Texas, for example, grand juries haven’t indicted a Houston police officer since 2004; in Dallas, grand juries reviewed 81 shootings between 2008 and 2012 and returned just one indictment. Separate research by Bowling Green State University criminologist Philip Stinson has found that officers are rarely charged in on-duty killings, although it didn’t look at grand jury indictments specifically.

When demonstrators and speakers—such as city councilperson Kshama Sawant and Pastor Carl Livingston—say Ferguson is "not an isolated incident," they're not just talking about other killings like Trayvon Martin's and Aiyana Jones's.

They're talking about a court system that that seems so tilted that, from a certain perspective, it's beginning to look weaponized.