The U.S. Justice Department has declined to press criminal charges in connection with a contract fraud investigation at a private Idaho prison.

The FBI began investigating the Idaho Correctional Center last year. The facility had been run by Corrections Corporation of America and was known for being so violent that inmates dubbed it "Gladiator School."

U.S. Attorney Wendy Olson said Wednesday that the probe was complete, and the agency didn't find probable cause to file charges.

The Idaho State Police Department was originally asked to investigate operations at the prison in 2013 after an Associated Press investigation showed the company was giving state officials falsified documents to cover up thousands of hours of understaffing.

The FBI took over after the probe failed to launch for more than a year.