Updated

A federal judge has taken the unusual step of asking prosecutors to investigate whether Denver police tried to intimidate a witness in a jail-abuse suit against the city.

The U.S. Attorney's Office hasn't said whether it will pursue Judge John Kane's request.

The latest allegations against the police and sheriff's departments came as part of a civil rights lawsuit filed in Denver by former inmate Jamal Hunter, who says a sheriff's deputy not only failed to protect him during a July 2011 beating by fellow inmates but encouraged the attack.

Experts say such inquiries rarely yield criminal charges.

Nancy Leong, an associate professor at the University of Denver's Sturm College of Law, says the cases are a challenge for prosecutors, who have to prove not just that an officer violated someone's civil rights but that they intended to.