OSHKOSH, Wis. (AP) — Newly released documents show a nearly two-week lockdown at a Wisconsin state prison more than 50 miles southwest of Green Bay was the result of illegal drug activity involving dozens of inmates.

WLUK-TV reports that about 70 correctional officers from five other prisons helped search the Oshkosh Correctional Institution for contraband during the lockdown that began June 1.

Documents show 30 inmates were identified for alleged drug involvement. They were found with a synthetic opioid called fentanyl, which doctors say can be 50 times stronger than heroin.

The entire prison was searched during the lockdown, including its housing units, laundry facilities, food service area and prison grounds.

State Department of Corrections documents shows changes are being made to the prison visitation room and incoming mail.