Can police officers be prosecuted for excessive force?
Yes, indeed. Though prosecution isn’t necessarily commonplace, police officers can be held criminally liable for use of unreasonable force under both state and federal law. (A separate recourse for victims of police brutality is a civil lawsuit—to read about it, see Suing the Police for Excessive Force.)
Example: In 1991 four police officers were involved in the roadside beating of Rodney King, who had been stopped after a brief high-speed chase. The Los Angeles District Attorney charged them with several offenses, including assault with force likely to produce great bodily injury (Cal. Penal Code § 245) and unnecessary assault by a police officer (Cal. Penal Code § 149). A California jury acquitted the officers of all but one charge (the jury couldn’t agree on a verdict for the remaining charge). After the ensuing L.A. Riots, the United States government prosecuted the officers for the same incident under 18 U.S.C. § 242, which prohibits civil rights violations by government actors. A federal jury convicted two of the four officers; the judge sentenced them to two and a half years’ imprisonment. (The dual prosecution didn’t violate the prohibition against double jeopardy because distinct governments were involved.) (Koon v. United States, 518 U.S. 81 (1996).)