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.

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.