Bryan Titsworth / WFAA-TV

Dallas police said it was one of the most confusing crime scenes they'd ever investigated.

Bryan Titsworth / WFAA-TV

A Dallas police officer was shot and wounded.

DALLAS — Three people were killed and a Dallas police officer was in serious, but stable condition after a shootout on Stemmons Freeway early Sunday.

Investigators said it all started around 2:30 a.m. when officers responded to the scene of what they believed was a major car accident in the southbound lanes of Interstate 35E near the Commerce Street exit.

Senior Cpl. Jerry Poston and his partner arrived and saw two people they believed were accident victims. But as the officers approached them, Poston was shot. The 17-year veteran of the force was in serious but stable condition at Methodist Medical Center and was expected to recover from his wounds.

"We don't know what started this," said police spokesman Lt. Vernon Hale. "It appears that every citizen who responded to this thought they saw a major accident, attempted to go assist, and was shot."

Police said one carload of Good Samaritans managed to return fire after one of them was hit.

"It appears we may have three innocent people shot, but we don't know if they were related in any other way," Lt. Hale said. "At this point, the scene is so complex that we truly don't know whether we have one shooter or multiple shooters—or why they were shot or by who."

It was later determined that the shooting suspect was among the dead. "Quite frankly, the majority of our witnesses are shot and injured or deceased at this point," Lt. Hale added.

Several people at the scene were taken into custody, but no one had been arrested as of Sunday morning.

The freeway was shut down during the investigation.

WFAA-TV photojournalist Bryan Titsworth contributed to this report.