FILE- In this July 8, 2010 file photo, Wanda Johnson, center, mother of Oscar Grant, who was killed by transit officer Johannes Mehserle, speaks during a news conference in Los Angeles. Mehserle was convicted of involuntary manslaughter in the shooting death of, Oscar Grant, an unarmed man killed on an Oakland train platform in 2009. At least 400 people are killed by police officers in the United States every year, and while the circumstances of each case are different, one thing remains constant: In only a handful of instances do grand juries issue an indictment, concluding that the officer has committed a crime. (AP Photo/Jae C. Hong)