Before Michael Brown and John Crawford III and Eric Garner and Aiyana Stanley Jones and Akai Gurley there was Oscar Grant. The transit police killing of the 22-year-old at Oakland's Fruitvale Station on New Year's Day in 2009 sparked national outrage when video of it went viral. Finally, there was recent proof that black men face an outsize risk of death at the hands of law enforcement. For killing Grant, former BART police officer Johannes Mehserle was convicted for involuntary manslaughter and sentenced to two years in prison. He only served 11 months.

While covering the trial, my colleague Julianne Hing wondered how best to pursue justice for black victims of police killings. "Criminal prosecutions are a necessary salve for families who want personal accountability for their deepest losses and courts remain the most public venue to demand justice for police officers' violent behavior," she wrote. "But for many organizers and academics who work on police brutality issues, they are not the most effective. Prosecutions so often end in acquittal, for one--as the painful verdicts for the cops charged with attacking Amadou Diallo, Sean Bell, Abner Louima and Rodney King all illustrate this."

ProPublica analyzed the FBI's data from 2010--the year of Mehserle was convicted--through 2012 and found that young black men are 21 times more likely than their white peers to to be killed by police.

Yet it's extremely rare for officers to face indictments, much less receive lengthy prison sentences.In California, for example, a 2013 analysis of on-duty officer-involved shootings by The Center for Investigative Reporting found that since 2005, only three officers -- including Mehserle -- have been prosecuted.

What follows is a far-from-exhaustive, intensely human look at how the data bears out. The guiding question: Can law enforcement be held accountable in a justice system that's set up to support their actions?