The man who shot and severely beat an Arizona state trooper last week was a former member of the Mexican federal police who was in the country illegally, authorities said, reports the Associated Press .

He had rolled his car on Interstate 10 before he inexplicably attacked the officer who had stopped to help.

A passing motorist who witnessed the attack retrieved a handgun from his vehicle and fired two shots at Leonard Pennelas-Escobar after he refused an order to stop attacking Trooper Edward Andersson. When Pennelas-Escobar got up and resumed his assault on Andersson, the motorist fired a fatal shot at Pennelas-Escobar.

"He definitely kept him (Andersson) from having much more serious neurological injuries from this beating," Department of Public Safety Director Frank Milstead said.

Milstead called a news conference Monday to offer more detail on the chaotic scene, but he was unable to provide an explanation for why Pennelas-Escobar attacked the officer.

Andersson, a 27-year department veteran, suffered gunshot wounds to the right shoulder and chest. He underwent surgery and has since been released from the hospital.

The motorist who shot Pennelas-Escobar hasn't spoken out publicly about the shooting.