RIVERSIDE, Calif. -

A Riverside police officer gunned down execution-style -- with his own handgun -- while trying to catch a suspected truck thief begged for the shooter not to kill him, a witness said on Frday.

Stephen McQueen of Riverside is the only known eyewitness to the slaying of 27-year-old Officer Ryan Patrick Bonaminio last Nov. 7.

McQueen was among several individuals called to testify today in the preliminary hearing for Earl Ellis Green, a 45-year-old ex-con accused of murdering Bonaminio.

A judge ruled late in the afternoon that Green will go to trial for the murder.

McQueen recalled that on the night of the attack, he was standing next to his Nissan Altima in the parking lot of the Community Care Center on Ridge Road, immediately adjacent to Fairmount Park.

The itinerant, who works as a grounds keeper for the rehab facility, testified that he was waiting for his girlfriend when he heard someone shout "Stop, stop, stop!"

According to the witness, he suddenly saw a man run through the dimly lit parking lot, with a police officer about 10 feet behind him.

"I heard the officer say, `Let me see your hands! Let me see your hands!"' McQueen recalled.

He said the fleeing suspect abruptly turned at the corner of the building, beneath a staircase, disappearing from sight. The police officer attempted to make the same turn but lost his footing in a planter filled with roses and landed on his back.

"There was some mud from the sprinklers," McQueen explained.

As he watched from around 50 feet away, the witness said the suspect immediately ran over to the officer and struck him three times in the upper body with what appeared to be a "pipe or a stick."

According to McQueen, the assailant then reared up with both of his hands around a dark-colored object, aiming at the officer. Bonaminio struggled to his feet with his hands in front of his face, palms toward the attacker, McQueen said.

"And he says, `Don't do it! Don't do it!"' the witness recalled.

He said the assailant fired three shots, two of which struck the policeman in the face and head, causing him to collapse face first.

McQueen said the suspect, a black man in dark clothing whose face he never got a good look at, fled back in the direction from which he had come, jumped into a yellow-colored semi truck with no trailer, and drove away.

Green could face the death penalty if convicted of first-degree murder, vehicle theft, being a convicted felon in possession of a gun and special circumstance allegations of killing a peace officer and committing a murder to avoid arrest.

Green has arrests and convictions that span more than two decades.

Bail-setting affidavits filed by Riverside police Detective Ron Sanfilippo detail what allegedly transpired on the night Bonaminio was gunned down.

According to the documents, while allegedly driving a stolen big rig with no trailer, Green got into a fender-bender with a female motorist near Market Street and the Pomona (60) Freeway. The woman summoned police when the defendant fled the scene.

According to Sanfilippo, Bonaminio stopped the truck on Market Street, near Ridge Road, where Green jumped out and fled into the park, with the officer and Iraq War veteran in pursuit.

A dashboard camera mounted on the slain officer's patrol unit captured a man purported to be Green running away from Bonaminio.

The four-year police veteran was discovered moments later by backup officers and rushed to Riverside Community Hospital, where he was pronounced dead.

Green fled the area in the stolen truck, according to police. The Rubidoux man was identified through a fingerprint match, leading to his arrest outside a Target store in Riverside two days later.

Search warrants executed at his residence at 5161 34th St. and another property led to the recovery of the slain officer's .40 caliber Glock pistol, authorities said.

Bonaminio was the first Riverside police officer to be killed in the line of duty since January 2001.