A gunman is on the run after killing two men on a Houston freeway as horrified motorists watched during the Thursday evening rush hour, authorities said.

Police initially got a call of an accident at 5:55 p.m., but when they arrived seven minutes later, they found a shooting scene.

The fatal incident started as a crash involving two cars headed eastbound on Interstate 10, Houston Police Assistant Chief Bobby Dobbins said Thursday evening at a news conference.

"One of the vehicles struck the other vehicle and spun it out ... similar to a pit maneuver," he said. A pit maneuver is a police tactic used to spin out a car from behind during a chase.

Gunman fired as the car rolled downhill

Two occupants of the vehicle that rammed the other car got out, Dobbins said. One had a weapon.

The gunman, described by a witness as a Hispanic male, fired multiple shots at the car that had been rammed, pursuing it as it rolled down a hill, he said. Witnesses said the shooter is possibly in his early 20s, thin build, about 5 feet 7 inches tall and was wearing a red shirt, according to a Friday statement from the police.

Police found narcotics in the car of the two victims, Dobbins said. The Harris County Institute of Forensic Sciences has not yet verified the identity of the men, who are ages 25 and 33, police said.

The shooter's motive is unknown, said Dobbins: "Road rage? An accident? Narcotics? We just don't know, yet so I won't speculate."

Houston Police Chief Art Acevedo tweeted Friday that "there is a high probability the murders are drug related."

One witness, who had a pistol in his car, fired several rounds when he saw the shooter turn toward him, Dobbins said.

It's unclear whether the first gunman was struck by a bullet. He ran back up the hill, got into his car and sped away, Dobbins said.

"We don't have a good description of the (shooter's) vehicle," he said. The four-door sedan is a newer model, dark colored, with unknown license plates, police said.

Journalist describes an 'unreal scene'

Daniel Gotera, a journalist for local station KHOU, heard the gunshots and saw people fleeing, he said.

"Unreal scene just witnessed on I-10 in Houston," he tweeted. "A guy gets out of his car with gun just shooting in the middle of freeway. Unbelievable."

Witnesses provided CPR to the car's occupants until authorities arrived. Houston police urge anyone who was in the area at the time to come forward.

That part of I-10 remained closed midmorning Friday for investigation.

CNN's Susan Scutti contributed to this report.