The state has opened an outbreak investigation after three COVID-19 cases were confirmed at the state prison in Windham.

The three Department of Corrections workers were tested on Oct. 21 and are in isolation now, Maine Center for Disease Control and Protection officials said.

They were among five confirmed cases out of more than 1,200 inmate and staff tests sent to the Maine Health and Environmental Testing Laboratory on that day. The other two positives were Maine State Prison employees, officials said.

One positive case, at a prison in Warren, was announced on Oct. 19.

State prisons and jails have been of special interest because their congregate living conditions are ripe for viral invasion. Twenty-eight prisoners went on hunger strike at the Auburn county jail on Oct. 13 in protest of a lack of coronavirus safety measures, and findings from Department of Corrections staff showed the state’s 15 county jails were failing to enforce basic precautions in late September .

At the time, the corrections worker in Warren was the third associated with the department to get the virus since March — one at the Mountain View Correctional Facility in Charleston and the other at the Bolduc Correctional Facility in Warren.

Since the onset of the virus, five adult prisoners — all at the Maine Correctional Center in Windham — and a juvenile at Long Creek Youth Detention Center in South Portland have tested positive, officials have said.

More test results are expected from Windham this week. All state prisons have updated their COVID-19 protocols to ensure staff and resident safety at all facilities, officials said.