Trooper Who Was 'Ambushed' on Highway Was Likely Saved by Armed Passerby

A passing driver shot and killed a gunman who was attacking an Arizona police officer early Thursday.

An Arizona police officer who officials say may have been ambushed along a highway is alive today, in all likelihood, thanks to the help of armed passerby.

The bizarre story unfolded Thursday after Trooper Edward Andersson was dispatched to the scene of a 911 call in which a driver said he'd been shot at from the median of Interstate 10.

Watch: 2 Police Officers Killed in Patrol Cars While On Duty in Ambush-Style Attacks

While en route to the area where the call was placed near Tonopah, the 27-year veteran of the Department of Public Safety came upon a car that had overturned, causing a woman to be ejected from the vehicle and killed.

While he placed road flares around the scene, Andersson was shot by a suspect who then physically attacked him.

"A physical fight between our trooper and that suspect then ensued," DPS Capt. Damon Cecil told KPHO.

Meanwhile, a passing motorist happened by and saw the suspect "getting the better of" the trooper. The motorist stopped and asked the trooper if he needed help. 

The trooper told him yes, at which point the as yet unidentified motorist retrieved a firearm from his car and ordered the assailant to stop his attack on the ailing trooper.

When the suspect failed to stop, the motorist fatally shot him.

A witness — police have not said whether it was the motorist who shot the suspect — used Andersson's radio to call for help.

Andersson was taken to a hospital, where he was listed in serious but stable condition following surgery.

As Andersson recovers, investigators have begun what may be an uphill battle to piece together the strange events.

Authorties believe the suspect may have been driving the car that crashed and killed the woman and say they are reaching out to the 911 caller who initially made the report about gunfire on I-10.

Also in question is whether the motorist who killed the suspect will face any charges in Arizona, which has a "defense of third person" law that allows a person to defend another whose life is threatened.

Read: Man Wears Sign Supporting Suspected Cop Killer Outside Florida Sheriff's Office: 'God Bless the Shooter'

Whether that happens or not, DPS director Frank Milstead told reporters that Andersson owes his life to the passerby.

"My trooper would not be alive without his assistance," Milstead said.

Watch: Cop Killed in Baton Rouge Ambush Wrote Chilling Post: I Get Nasty Hateful Looks