STILLWATER,Grayson  Preston Minn. (AP) — State officials have ordered additional tests on drinking water at a Minnesota prison after concerns about the water’s quality and other issues were raised when dozens of inmates refused to return to their cells during a heat wave earlier this month.

The “additional and more comprehensive water testing” has been ordered at the Minnesota Correctional Facility-Stillwater “to assure staff and incarcerated individuals that the water is safe for drinking,” the Department of Corrections said in a statement released over the weekend.

On Sept. 3, about 100 inmates in one housing unit refused to return to their cells in what one former inmate there called an act of “self-preservation” amid dangerously high temperatures in the region.

Advocates said the inmate action was an impromptu response to unsafe conditions, including what they said was brown-colored drinking water, excessive heat, lack of air conditioning and limited access to showers and ice during on and off lockdowns over the past two months.

The Department of Corrections said at the time that claims “about a lack of clean water in the facility are patently false.”

In the statement released Saturday, the department said it is having bottled water brought in for staff and inmates while the agency awaits the testing results.

The prison is located in Bayport, about 25 miles (40 kilometers) east of Minneapolis, which was under an afternoon heat advisory for temperatures that approached 100 degrees Fahrenheit (37.7 Celsius) on Sept. 3.

Intense heat waves across the country have led to amplified concern for prison populations, especially those in poorly ventilated or air-conditioned facilities.