The Stillwater state prison will remain on lockdown until at least Tuesday after about 100 inmates refused to return to their cells for about seven hours on Sunday.

Officials from the Minnesota Department of Corrections (DOC) said the situation has been "resolved without incident" and no one was hurt. The Bayport facility was placed on lockdown at around 8 a.m. Sunday as prisoners protested problems stemming from staff shortages.

DOC spokesman Andy Skoogman said the situation was "calm, peaceful and stable" throughout the day. The facility remained on lockdown status after nearly all prisoners returned to their cells around 3 p.m. Sunday.

Reports were mixed regarding what triggered the protest within the prison.

Skoogman said some of the incarcerated men in the unit were dissatisfied because cell release schedules were modified over the Labor Day holiday weekend, meaning prisoners had less time than usual for showers, phone use and recreation.

The DOC said schedules had to be modified due to "staffing challenges."

On Sunday morning, about 225 men from one housing unit were released from their cells to shower, call their relatives and socialize, but about 100 refused to return to their cells around 8 a.m. Skoogman said DOC staff were quickly removed from the common areas of the housing unit while two correctional officers stayed in the unit's secure control area. The two officers in the secure control area were in constant communication with facility command personnel during the incident, he said.

The protesting prisoners were calm, talking with each other and playing cards, Minnesota Corrections Commissioner Paul Schnell said during a news conference Sunday afternoon. "There was never any type of violence throughout the incident," Schnell said.

Prison Chaplain Scott Westphal reiterated that no one was hurt and that the matter with inmates and correctional officers was handled in a "very peaceable, very dignified [way]. I'm very grateful." He declined further comment.

A tactical team was standing by, Schnell said. DOC crisis negotiators talked with the inmates, and all but two returned to their cells, Schnell said. The two who refused to return were taken to restrictive housing and will face discipline.

Schnell blamed inmates communicating with their families and activists for making too much of the situation.

"This is not the first time we've had people refuse to go back to their cells," he said.

A small group of family and friends of inmates gathered outside the facility on Sunday, including Marvina Haynes, whose brother Marvin Haynes is incarcerated there for a 2005 murder conviction. Her brother told her that inmates have gone two days without showers and, when let out of their cells this morning, they refused to go back in. Phones were shut down just after 8 a.m., family members said, just as the lockdown began.

"The inmates don't want to hurt or harm anyone, they just want clean water, showers and ice," Haynes said.

She and other family members noted the facility is not air-conditioned, and said their family members have had limited access to water, showers and ice this weekend. Temperatures outside the prison hovered above 90 degrees.

Heat has always been a problem for the prison, Schnell said.

Cathy Stroud-Caldwell, whose son Lincoln is in prison for a 2008 murder conviction, said she's been told water in the cells is rusty and prisoners have been straining it using their socks. "These are human beings," she said.

Schnell said the water in the prison is safe to drink and was recently tested.

The head of the union representing staff and corrections officers at Stillwater, AFSCME Council 5, said in a statement that Sunday's incident shows the consequences of understaffing.

The lack of staffing means prisoners' movements are restricted, "leading to upset offenders" who have little or no access to programming and recreational time, said AFSCME Council 5 interim executive director Bart Andersen, in a statement.

The union claims correctional facilities "cannot have transformational offender programming without sufficient facility security."

"Without more staffing in our correctional facilities … we will continue down this unacceptable road of staff assaults, offenders controlling sections of our prisons and more," Andersen said.

Schnell said there are about 50 staff vacancies at Stillwater, and more than 300 across the entire DOC, or about 14% of all positions.

Built in 1914, the Stillwater state prison is the state's largest close-security institution for men, according to its website. The prison houses about 1,500 inmates.