WASHINGTON (AP) — As the Justice Department probes the police shooting of an unarmed 18-year-old in Ferguson, Missouri, history suggests there's no guarantee of a criminal prosecution, let alone a conviction.

Federal authorities investigating possible civil rights violations in the Aug. 9 death of Michael Brown must meet a difficult standard of proof. It's a challenge that has complicated the path to prosecution in past high-profile police shootings.

To build a case, the Justice Department would need to establish that officer Darren Wilson not only acted with excessive force but also willfully violated Brown's constitutional rights — a standard experts say is difficult to meet and is complicated by the fact that officers are given latitude in their use of force.

A county grand jury is also hearing evidence in its own investigation.