STILLWATER — A Minnesota prison has “resolved without incident” a situation involving about 100 inmates in one housing unit who would not return to their cells Sunday in what one former inmate there called an act of “self-preservation” amid dangerously high temperatures in the region.

The situation was “calm, peaceful and stable throughout the day,” a Department of Corrections spokesperson said in a statement, adding that “incarcerated individuals in the unit indicated dissatisfaction” because the understaffed facility had to limit inmates’ time out of their cells.

But advocates positioned outside of the Stillwater prison, some of whom have family members inside, said inmates are fed up with the excessive heat, lack of air conditioning and limited access to showers and ice during on and off lockdowns over the past two months.

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

“My organization got calls from inmates who are actually inside” starting at 6:30 a.m., said Marvina Haynes of Minnesota Wrongfully Convicted Judicial Reform, whose brother is an inmate at Stillwater.

“This morning, they decided that they weren’t going to lock into their cells,” said David Boehnke of Twin Cities Incarcerated Workers Organizing Committee.

The department confirmed that inmates have been in lockdown status because of the holiday weekend, meaning they are kept in their cells, with “limited access facility-wide to out-of-cell time for showers, phone use and recreation.” The facility remains on lockdown now, and all inmates have returned to the cells.

The executive director of the union representing Stillwater’s correctional officers, Bart Andersen, said in a statement that the incident is “endemic and highlights the truth behind the operations of the MN Department of Corrections with chronic understaffing.”

Andersen said such conditions upset inmates because of restrictions on program and recreation time “when there are not enough security staff to protect the facility.”

Haynes, Boehnke and Cathy Stroud Caldwell said the inmate action was an impromptu response to unsafe conditions, including access to clean drinking water, which they say is reportedly brown in color. The department said those claims “about a lack of clean water in the facility are patently false.”

“They didn’t have time to organize and plan,” Haynes said. “It was just … we’re not going back to that hot cell with no drinking water and not being able to shower.”

Haynes said they hope to meet with officials “to talk about the conditions that inmates are living in” and “solutions for the future.”

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

Two correctional officers stayed in a secure control area and in contact with facility staff since the emergency lockdown status was initiated at 8 a.m. There were no injuries, according to the Department of Corrections.

Members of a crisis negotiation team and the Special Operations Response Team were deployed “out of an abundance of caution.”

In total, about 1,200 inmates are at the facility just southeast of Stillwater in Bayport, according to department records. It was built in 1914.

Kevin Reese, founder of a criminal justice organization, Until We Are All Free, described Stillwater as a “pizza oven” in the summers. He was incarcerated there during the summers from 2006 through 2009.

“It is a 100-year-old building with no air conditioning, no central air,” Reese said. “The walls actually sweat.”