MADISON, Wis. (AP) — Violence by inmates at Wisconsin's juvenile prison sent five staff members to the hospital Sunday night in apparently coordinated attacks that workers fear may have been practice for a full-fledged riot, a retired union steward said Tuesday.

After news of the latest unrest broke, Gov. Scott Walker ordered the head of Wisconsin's prison system to put an interim juvenile prison superintendent in place. And a federal judge ordered that he be given an update on conditions at the prison by Nov. 10.

"I will not ignore the safety of either the staff or the youths at Lincoln Hills and Copper Lake Schools," U.S. District Judge James Peterson said in a letter to a pair of state lawmakers who wrote him last week and raised concerns about increased violence against guards.

Doug Curtis, the former Lincoln Hills union leader who spoke with staff about what happened Sunday, said workers feared that inmates were practicing for a larger disruption.

Tristan Cook, spokesman for the state Department of Corrections, disputed the characterization of the disturbance as "testing" of how prison staff would respond.

"Youth confined in their rooms were damaging state property," he said in a statement. "A number of staff responded and subdued the disruptive youth."

Two weeks ago, a teacher was knocked out and sent to the hospital after being punched by an inmate. The Milwaukee Journal Sentinel first reported on Sunday's violence, saying a pair of inmate-on-staff assaults occurred including an inmate hurling pieces of a plastic chair at a guard.

Curtis said he did not know the extent of the injuries from Sunday's attack, but that one employee may need knee surgery.

"It's on and on up there," Curtis said of the juvenile prison in Irma, about 30 miles north of Wausau. "It's happening almost daily."

The Lincoln Hills-Copper Lake prison complex, which houses about 160 girls and boys in northern Wisconsin, has been under federal investigation for alleged prisoner abuse and child neglect for nearly three years. No one has been charged. Numerous lawsuits have been filed, though, including one by teenage inmates challenging the use of pepper spray and other disciplinary tactics at the prisons.

Peterson, the federal judge, in July ordered the Department of Corrections to scale back its use of pepper spray, solitary confinement and shackles at the prison, saying they amounted to unconstitutional cruel and unusual punishment.

But since then, a string of violent attacks on prison guards have been reported both by workers at the facilities and the two state lawmakers who last week asked the judge to reverse his ruling. The lawmakers and prison workers say conditions have worsened since the court order because inmates feel emboldened.

Walker's administration told the judge earlier this month that it has not yet fully complied with the order because of ongoing "significant unrest" at the prison.

On Tuesday, Walker ordered Department of Corrections Secretary Jon Litscher to appoint an interim juvenile prison superintendent while the search for a permanent one continues. The job has been vacant since Sept. 1. Just last week Litscher said the juvenile prison complex was safe for both guards and inmates.

"We cannot allow individuals convicted of battery, armed robbery, and even murder to feel empowered to attack staff and each other in these institutions," Walker said in the letter ordering Litscher to make the appointment. "No matter the challenges, we must provide the men and women working in CLS/LHS with the tools they need to maintain order in the facilities and protect themselves and to protect the offenders held at these institutions."

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