"Generally, the Justice Department is reluctant to get involved in cases that have already been tried before a state jury, in part because of concerns about double jeopardy.
Perhaps the best-known example where federal prosecutors did intervene was the case of four police officers acquitted after a California state trial in the beating of motorist Rodney King, which triggered deadly riots in the Los Angeles area in 1992.
Two of the four officers were convicted in federal court of violating King's rights, but that case differs from Zimmerman's because they were acting as sworn law enforcement officials, not as a private citizen claiming self-defense.
In contrast, the Justice Department declined to prosecute New York Police Department officers after they were acquitted in the 2006 shootings of three men including Sean Bell, who was fatally wounded the morning of his planned wedding. The short Justice Department statement — issued in 2010, four years after the shooting — simply said there was insufficient evidence to proceed.
"Neither accident, mistake, fear, negligence nor bad judgment is sufficient to establish a federal criminal civil rights violation," the department said in the Bell case."