PHOENIX — An Arizona state trooper was shot in an ambush by a man who then bashed the officer’s head against the pavement until a passing driver shot the suspect to death, authorities said.

The trooper suffered a severe wound to his shoulder and upper chest but he is expected to recover at a hospital.

‘‘My trooper would not be alive without his assistance,’’ Department of Public Safety director Ralph Milstead said of the good Samaritan.

The drama unfolded at an early morning rollover wreck on Interstate 10 in the desert west of Phoenix in which a woman was ejected and killed. Authorities believe the man who shot the officer was driving the car that crashed.

The officer was putting out flares when the suspect opened fire, Milstead said. The man jumped on top of the wounded officer and slammed the trooper’s head against the pavement, Milstead said.

When the passing driver asked if the trooper needed help, he said yes, and the passerby went back to his vehicle, got his gun, and told the suspect to stop the attack.

When he didn’t, the driver shot him.

Associated Press