Inmates at Stillwater prison were restricted to their housing units Friday and visitation by friends and family was canceled until further notice, a spokesman for the Minnesota Department of Corrections said.

The "ongoing security procedure" didn't result from a disturbance but rather gives prison staff time to fulfill other responsibilities, said John Schadl, the department's communications director.

Stillwater prison, housing about 1,600 men and many of them violent offenders, is Minnesota's largest close-custody prison.

During the lockdown, prisoners won't have access to the prison school, industries, chapel and other activities, Schadl said.

He said an ending date for the lockdown, which began Thursday, hasn't been decided but will be re-evaluated next week.

"It's an orderly process," he said, but declined to further describe the reasons for the lockdown.

Stillwater prison occasionally imposes a lockdown after incidents of violence. That happened in February when an inmate attacked a female correctional officer and a contract food service employee, beating them in the prison's dining hall about midday. Additional officers raced to the scene and the offender was subdued.

The number of offender-on-staff assaults in Minnesota's 10 prisons has dropped sharply over the past two years, Corrections Commissioner Tom Roy said at the time. In 2010, there were 42 such incidents. In 2011 there were 16, and in 2012 there were 17, he said.