A St. Louis County grand jury on Monday decided not to indict Ferguson, Missouri, police officer Darren Wilson in the August killing of teenager Michael Brown. The decision wasn’t a surprise — leaks from the grand jury had led most observers to conclude an indictment was unlikely — but it was unusual. Grand juries nearly always decide to indict.

Or at least, they nearly always do so in cases that don’t involve police officers.

Former New York state Chief Judge Sol Wachtler famously remarked that a prosecutor could persuade a grand jury to “indict a ham sandwich.” The data suggests he was barely exaggerating: According to the Bureau of Justice Statistics, U.S. attorneys prosecuted 162,000 federal cases in 2010, the most recent year for which we have data. Grand juries declined to return an indictment in 11 of them.

Wilson’s case was heard in state court, not federal, so the numbers aren’t directly comparable. Unlike in federal court, most states, including Missouri, allow prosecutors to bring charges via a preliminary hearing in front of a judge instead of through a grand jury indictment. That means many routine cases never go before a grand jury. Still, legal experts agree that, at any level, it is extremely rare for prosecutors to fail to win an indictment.

“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.

There are at least three possible explanations as to why grand juries are so much less likely to indict police officers. The first is juror bias: Perhaps jurors tend to trust police officer and believe their decisions to use violence are justified, even when the evidence says otherwise. The second is prosecutorial bias: Perhaps prosecutors, who depend on police as they work on criminal cases, tend to present a less compelling case against officers, whether consciously or unconsciously.

The third possible explanation is more benign. Ordinarily, prosecutors only bring a case if they think they can get an indictment. But in high-profile cases such as police shootings, they may feel public pressure to bring charges even if they think they have a weak case.

“The prosecutor in this case didn’t really have a choice about whether he would bring this to a grand jury,” Ben Trachtenberg, a University of Missouri law professor, said of the Brown case. “It’s almost impossible to imagine a prosecutor saying the evidence is so scanty that I’m not even going to bring this before a grand jury.”

The explanations aren’t mutually exclusive. It’s possible, for example, that the evidence against Wilson was relatively weak, but that jurors were also more likely than normal to give him the benefit of the doubt. St. Louis County prosecutor Robert McCulloch has said he plans to release the evidence collected in the case, which would give the public a chance to evaluate whether justice was served here. But beyond Ferguson, we won’t know without better data why grand juries are so reluctant to indict police officers.