A federal investigation into alleged civil rights violations at Wisconsin's troubled youth prison has ended without charges, prosecutors announced Friday.

The U.S. attorney's office in Madison said an investigation by the FBI and the U.S. Department of Justice's Civil Rights Division found insufficient evidence that staff members at Lincoln Hills School had used unreasonable force against inmates repeatedly. The attorney's office said in a statement that prosecutors faced a heavy burden in proving that staff members willfully used more force than necessary.

"Federal prosecutors must not only prove that the force was excessive, but must also prove, beyond a reasonable doubt, that the staff member acted with the deliberate and specific intent to do something the law forbids," the statement said. "In this instance, there was insufficient evidence to establish beyond a reasonable doubt a violation of the federal criminal civil rights statutes."

Problems at the youth prison outside Irma in northern Wisconsin have been building for years. Workers say conditions got worse in 2011 when two juvenile prisons near Milwaukee closed and teens were consolidated at the facility. The FBI launched the probe in 2015.

The prison also has been the subject of multiple lawsuits, including one that resulted in a federal judge ordering sweeping reductions in the use of pepper spray, solitary confinement and shackles on juveniles.

Valerie Landowski, a spokeswoman for American Federation of State, County and Municipal Employees Local 6, which represents guards at the prison, said the union is satisfied with the a decision not to prosecute anyone. She said conditions at the prison have been deteriorating because former Gov.-Scott Walker's signature law stripping public workers of their collective bargaining rights has prevented the guards from negotiating to improve morale and safety, leaving them overworked.

"Whether or not they're exonerated in this particular case, we'll let the investigation speak for itself," Landowski said.

Walker signed a bill last year that requires the Department of Corrections to close the prison by 2021 and allows the agency to borrow up to $40 million to build smaller regional facilities to house serious juvenile offenders and expand the Mendota Juvenile Treatment Center in Dane County.

Current Democratic Gov. Tony Evers has said the closure timeline is too aggressive. He wants to keep the prison open indefinitely until replacement facilities are built. The corrections department announced last month that it plans to build two new facilities in Milwaukee and Outagamie counties to house serious juvenile offenders.

Republican state Rep. Michael Schraa, chairman of the Assembly's corrections committee and one of the bill's chief sponsors, said he was pleased to hear the federal investigation had ended without turning up enough evidence to prosecute anyone.

"It was a black cloud hanging over the (Department of Corrections) for a long time," he said. "When we visited the (youth prison) two years ago, that was one of the things they said, they felt like they were walking on eggshells. They didn't know when the hammer was going to fall. Now that it is concluded and there's no indication excessive force was used, I think the anxiety level for the (guards) will go down. That makes a better environment."

Sen. Lena Taylor, a Milwaukee Democrat who sits on the Senate judiciary committee, said in a statement that she doesn't have "a ton of confidence" in the information provided to investigators.

"We know that something was very wrong at Lincoln Hills," she said. "There are lawsuits, injured youth and records of excessive use of pepper spray and solitary confinement to support that belief."

Evers' spokeswoman, Melissa Baldauff, issued a statement saying the governor looks forward to working to get juvenile inmates the support they need in settings closer to home.

Cass Bowers, the communications director for the Wisconsin chapter of the American Civil Liberties Union, one of the groups that filed the lawsuit resulting in the reduction in the use of pepper spray and solitary confinement. He said in an email that the investigation doesn't affect their case but a decision not to file charges "doesn't amount to an endorsement of the practices that allegedly took place back before 2015."

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