Before Monday's night announcement that a grand jury had voted not to indict Darren Wilson in the shooting death of Michael Brown, two facts were in tension:
1. Grand juries, at least at the federal level, almost always vote to indict people accused by prosecutors of a crime. As you can see in the chart above, federal prosecutors pursued over 160,000 cases against defendants in 2009-2010 (the last period for which there is data), and grand juries only voted not to return an indictment in 11. Prosecutors decided not to pursue charges in thousands of cases for a host of other reasons, such as weak evidence or other authorities were pursuing the case.
2. Grand juries, at least at the state level, often do not indict police officers. As Ben Casselman of 538, who made the observation about the rarity of grand juries not indicting, noted, "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. ... 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."
Of course, what would be best to know is the frequency of grand jury indictments at the state or local level. But the governing law, established by the Supreme Court in the 1980s, allows police officers to shoot to kill as long as they believe their life is in imminent danger — which Wilson has said he believes was the case. Given that, many were skeptical that the grand jury was going to indict Wilson, even if the only threshold was "probable cause."