A Colorado prison is now the site of the largest known outbreak of COVID-19.

At least 138 inmates at the state’s biggest prison, Sterling Correctional Facility have tested positive for the virus, The Denver Post reports. On Wednesday, 473 inmates were tested, with 255 results arriving on Friday. Of those tests, 104 were negative, 12 were inconclusive, and one result was unsatisfactory. Two department employees had contracted coronavirus prior to the mass testing, which didn’t include staff. The Colorado Department of Corrections started the expansive testing after eight inmates tested positive.

“Given the insidious nature of this virus we had suspected that despite seeing a relatively low number of inmates with symptoms, the number of positives was potentially much higher,” Department of Corrections Executive Director Dean Williams said in a news release. “That is exactly why we conducted this large scale testing, so that we can continue to isolate, monitor and treat any inmates who were positive and try to mitigate the spread to others inside the facility.”

The facility initiated a lockdown on April 14. Since then, inmates have eaten meals in their cells and can only leave to use the restroom or shower. Both staff and inmates have been instructed to wear masks and certain inmates are having their temperatures read twice a day.

The privately-owned, Colorado-based Bent County Correctional Facility has also instituted a lockdown after a staff member tested positive for the virus on April 16. Denver Women’s Correctional Facility, the Colorado Territorial Correctional Facility, and the Arkansas Valley Correctional Facility haven’t authorized such strict measures.

Colorado Gov. Jared Polis has requested that the DOC release 500 to 700 inmates to stop prison overcrowding and close part of a facility. He then plans to use staff to reopen a vacant prison, where new inmates can be quarantined for 14 days.

Prisons have been hit hard by the pandemic. According to The Marshall Project, Over 140 staff and inmates across U.S. federal and state prisons have died from the virus and more than 9,000 have been infected. As of Saturday, over 200,000 Americans have perished from COVID-19.