Topeka ? The inmate population at a low-security state prison in northwestern Kansas grew steadily in the three months before a disturbance left part of a housing unit there unlivable and prompted the state to transfer nearly 12 percent of its prisoners Wednesday.

State Department of Corrections spokesman Samir Arif said the department moved 100 of the 856 inmates at the Norton Correctional Facility out after Tuesday night’s disturbance was contained. He said it started when an inmate or inmates set fire to a mattress in a housing unit and as many as 250 spilled into the yard.

The disturbance in Norton, about 320 miles (515 kilometers) west of Kansas City, followed multiple disturbances in recent months at another, maximum-security prison in southern Kansas. Robert Choromanski, executive director of the Kansas Organization of State Employees, which represents corrections officers, called it a major riot, echoing a tweet Tuesday night.

Daily department reports available online show that the inmate population in Norton grew steadily, up 21 percent on Tuesday from the 707 inmates housed there at the end of May. Arif said the state began double-bunking inmates in parts of the prison this summer; its department-set capacity jumped from 707 to 902 beds in late June.

The state’s maximum-security prison in El Dorado, about 30 miles (48 kilometers) northeast of Wichita in southern Kansas, also saw its population rise steadily and increased double-bunking before a disturbance in late June. It was among multiple episodes of unrest in recent months that prompted Republican Gov. Sam Brownback to grant immediate pay raises for corrections officers last month.

“We have a self-inflicted crisis in the Department of Corrections caused by rapid, often questionable, relocation of inmates among facilities,” state Sen. Laura Kelly, of Topeka, the top Democrat on the Senate budget committee, said in an email. “It’s not safe or fair to either the inmates or the staff.”

Arif declined to respond to Kelly’s comments. He said the department is investigating to determine what started Tuesday night’s disturbance and how many inmates were involved.

He said inmates set small fires, smashed windows in several buildings and broke into a tool shed, though the department reported only two minor injuries to staff that did not require medical attention. Local law enforcement officers and firefighters went to the prison, though the department had its own firefighting crew, escorted in by sheriff’s deputies.

“There was mass confusion out there,” Arif said Wednesday. “Pretty much everybody was out in the yard.”

Arif said that even with the damage that made part of one housing unit not livable, the 100 inmates were transferred out of the Norton prison as “a security thing, not a space thing.”

The Norton facility is the state’s western-most prison. The town of Norton, with about 2,800 residents, also is 12 miles (19 kilometers) south of the Nebraska state line.

It seemed an unlikely place for a disturbance. Almost all of its inmates are “low” medium-security inmates or minimum-security inmates, either close to finishing their sentences or sentenced to short prison terms.

Also, the Norton prison has a low employee turnover rate. As of Tuesday, 17 of its 196 uniformed-officer positions were open, or 8.7 percent, as compared with a 33 percent annual turnover rate among uniformed officers in the state prison system in general.

Yet the number of disciplinary reports on inmates at the Norton prison spiked in August at 396, up 75 percent from the 226 reports in July, according to figures that the department released to The Associated Press on Tuesday prior to the disturbance. Last year the prison averaged 209 reports a month, compared to 245 per month from January through August of this year.

Choromanski said he believes the unrest was stoked partly by inmates unhappy after being transferred from other prisons.

“It shows it can happen anywhere, in any facility,” Choromanski said.

The department previously confirmed three disturbances in May and June at the maximum-security El Dorado prison, as well as two inmate-on-inmate fights on July 28 that sent one inmate from each altercation to a hospital with stab wounds. The annual turnover among uniformed officers at that facility is 46 percent.

Corrections Secretary Joe Norwood has attributed the disturbances there to newly arrived inmates who were transferred from other prisons. But the unrest raised concerns among legislators about staffing shortages and pay for corrections officers.