WARREN – A string of inmate assaults at the Maine State Prison last week included an attack in which a prisoner was stabbed 17 to 19 times in the back and head.

The first of three assaults took place Thursday when an unidentified prisoner was stabbed with a homemade metal shank more than a foot long, Warden Jeffrey Merrill said Monday.

The 17 to 19 “superficial” wounds were to the prisoner’s back and the back of his head, Merrill said.

None of the three assaults required prisoners to be taken to an outside hospital, Merrill said, but medical treatment was administered in-house.

He did not indicate whether the assaults were connected.

The inmates’ names were not disclosed because all three were under investigation Monday, Merrill said.

Besides being charged in-house, inmates may face aggravated assault charges by the district attorney and, if convicted, those inmates could receive additional prison time, he said.

The stabbing Thursday occurred in the housing area when one inmate visited another in a cell, a violation of policy, Merrill said.

The most serious injuries took place Saturday morning when prisoners were released to the recreation area in the recreation building. That assault was captured on videotape by cameras at the prison and is being reviewed by administration.

In that incident, one inmate was attacked by a group of 15 prisoners and was “severely assaulted with a padlock in a sock,” Merrill said. “It seems to be a common thing,” he added, referring to the weapon.

“He was beaten pretty badly,” Merrill said, noting the unidentified prisoner required numerous stitches.

Padlocks are issued to prisoners to secure footlockers where they store their belongings. Inmates are encouraged to use the locks so that other inmates do not steal their property, Merrill said.

“This one was premeditated,” Merrill said. “This assault took place in a matter of seconds.”

Another prisoner, who intervened in the mob attack, also was hurt, but not seriously, Merrill said.

On Friday, a prisoner sought medical attention for serious bruising.

That inmate was “obviously assaulted” by one or more people, Merrill said.

No staff members were injured during any of the attacks, he said.