AMESBURY (CBS) – A State Police trooper was stabbed by a suspect while he was working a highway detail on Route 495 in Amesbury. The suspect was shot and wounded by the trooper.

It happened on the southbound side of the highway around noon on Thursday, near Exit 54.

Massachusetts State Police Col. Christopher Mason said a preliminary investigation showed that the 34-year-old trooper was protecting a construction crew that was working on signage when the suspect, identified as 18-year-old Nathan Aguilar, of Melrose, allegedly pulled his van over into the breakdown lane, parked his vehicle and approached the trooper’s cruiser.

Mason said Aguilar allegedly came up to the driver’s side window with a knife in a black ski mask, and the trooper and Aguilar got into a struggle, resulting in the trooper sustaining knife wounds to his left arm. He said the trooper was forced to shoot Aguilar.

Mason called the radio call for back up from the trooper “chilling.” He said both Amesbury and State Police responded to the scene. They found the trooper with stab wounds and the suspect on the ground.

Aguilar was taken to a local hospital and then flown to Beth Israel hospital and is in custody. He is facing life-threatening injuries. The trooper was treated and later released from Lawrence General Hospital.

Mason said that there is currently no evidence that Aguilar knew the trooper, and that a motive is unclear at this time.

Police have yet to charge Aguilar with a crime, but said he will be charged.

“I think it speaks to the highly unpredictable, high threat level we police in these days,” Mason said.

All roads were reopened by 5:20 p.m.