Stillwater prison was on lockdown Sunday after two inmates assaulted and injured two corrections officers a day earlier, the Minnesota Department of Corrections (DOC) said.

The violence began about 11:30 a.m. Saturday when one inmate attacked an officer at a security post in a housing unit, the DOC said. When another officer intervened, a second inmate reportedly attacked them.

The officers were taken to a hospital, where they were treated for injuries and released. Their alleged assailants were taken to the maximum-security Oak Park Heights prison.

The Stillwater prison will remain on lockdown "to allow investigators to identify the motive for and events leading up to the assaults," DOC said.

The department said it will submit the investigation to the Washington County Attorney's Office for possible felony charges of assault on a corrections officer.

"This attack on correctional officers at the Stillwater correctional facility is another unfortunate reminder of the challenges and risks corrections staff face," Corrections Commissioner Paul Schnell said in the news release.

"We will pursue prosecution of the perpetrators of these cowardly attacks to the fullest extent of the law, and we recognize and appreciate the honorable actions of the incarcerated man who courageously intervened."

The head of the union representing the corrections officers called on the DOC and other authorities to conduct a thorough investigation.

Bart Andersen, executive director of AFSCME Council 5, said the investigation should include "credible reports that this attack was deliberate and planned."

The Incarcerated Workers Organizing Committee, a prison-reform activist group, was skeptical.

"In ten years of monitoring the prison system we have yet to hear of a staff assault that was not provoked by mistreatment or abuse on some level," read a statement from the group.