SPRINGFIELD (AP) –  An Illinois prison spokeswoman says an assault on six workers by five inmates at the maximum-security Pontiac Correctional Center appears to have resulted from a failure to follow workplace safety procedures.

Illinois Department of Corrections spokeswoman Nicole Wilson said Monday that the agency’s investigation of the incident will include looking into why procedures weren’t followed and how future incidents can be prevented.

Wilson says the assault doesn’t appear to have resulted from a lack of policy or a breakdown in existing policies. She was responding to a union statement that policies and procedures designed to keep staff safe have been ignored or superseded by new practices.

The American Federation of State, County and Municipal Employees says the assault Sunday was sparked by an inmate punching a correctional lieutenant.  The union says four correctional officers and two lieutenants at Pontiac Correctional Center were treated for abrasions, contusions and possible concussions suffered in the incident Sunday. They have all been released from the hospital.

Joe Lewis is a correctional officer at the facility and president of the AFSCME local that represents the prison workers. He says an inmate started punching the lieutenant and knocked her to ground, then other inmates joined the fray, injuring other employees who tried to help the lieutenant.

The Illinois Department of Corrections has put the facility on lockdown and is investigating.