The Good Samaritan is rattled by the realization that trying to help someone could have gotten him killed.

A 27-year-old Arvada man's voice tightened as he recounted watching a gunman shoot and kill a female car-crash victim Tuesday night.

As a sleepless night turned into an edgy day, the witness kept replaying what happened - and what might have happened - if he had rushed to the wrecked car instead of stopping to call 911.

For his safety, the witness asked that his name be withheld, because the suspected gunman, Tyler James Martin, remains at large. The suspect is the ex-boyfriend of the victim, Amber Cremeens, 34.

"One of the first things that crossed my mind is, I could have died. If I had more of a sense of urgency and I had ran up to the car, he could have very well killed me," the man said.

The witness said he was listening to music in his pickup as he drove south on Wadsworth Boulevard in Wheat Ridge about 10 p.m. Suddenly, he saw a black Dodge Caliber ahead him slowly weaving back and forth.

He vaguely noticed that a dark sedan was driving next to the Dodge.

Thinking the woman driver might be dozing off or even having a heart attack, the pickup driver slowed down.

The Dodge veered off the road, bouncing into the air as it hit the curb at 40 mph and slammed into a chain-link fence.

The witness pulled over about 40 yards north of the wrecked car, jumped out and began dialing 911 for an ambulance. He saw the dark sedan pull over next to the Dodge.

A stocky, balding man wearing dark-frame glasses got out and slowly walked around to the Dodge's driver's-side window.

"I thought that it was just somebody who was concerned," the witness said. "He reached into his front jacket pocket and I thought he was reaching for his cell phone."

As the Samaritan looked down at his own cell phone to redial 911, he heard a pop. He remembered thinking that the Dodge's tire must have blown.

Then he looked up and froze as the stocky man aimed a gun and fired two more shots point-blank at the woman's head as she sat in the car.

"I ran back and jumped into my truck," the man recounted.

His pickup's bright lights caught the gunman, still standing by the Dodge.

"He looked directly at me and then just looked at the woman. He had his arm extended and I could see his silver gun in his left hand," the witness said.

The witness said the gunman got back in his car and drove off.

Police say the gunman fired at the woman as she drove, and the witness suspects she was wounded when her car veered off the road.

Witnessing a killing on a busy suburban boulevard "makes me feel a sense of vulnerability that I never really had," the man said.

"I mean, the woman was shot as she was driving her car down Wadsworth, of all places. Like it could happen anywhere," he said.

Even hours after the shooting, the "what-ifs" wouldn't stop playing in the man's head.

"The lesson that I learned from it is if I ever witness an accident again - and I would pass this along to anybody - never exit the vehicle," the man cautioned. "Stay within a safe proximity until you can assess what happened. Because I just assumed that there was no level of hostility . . . and I just assumed wrong."