Texas law enforcement has launched a manhunt for a gunman and his accomplice after they allegedly shot two people dead on a freeway amid rush hour on Thursday night.

The shooter opened fire with an assault-style rifle on a vehicle that had struck his car before 6:00 p.m. on Interstate 10 in Houston, according to police.

Houston Police Department told reporters the gunman fired multiple rounds at the victim’s vehicle as it rolled backward down the highway and continued to shoot after the car stopped. The two men inside the vehicle were killed. Police later discovered drugs inside the car, said officers.

One person fired multiple rounds at the gunman, who fled the scene in his car, according to police. It’s unclear whether the gunman was hit.

KHOU-TV reporter Daniel Gotera said he heard the gunshots and saw drivers speeding away from the area.

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

The part of the freeway where the incident occurred was closed overnight as detectives investigated the crime scene.

It is unknown what the gunman’s motives were, police said.

“Road rage? An accident? Narcotics? We don’t know yet,” Houston Police Assistant Chief W.R. Dobbins said Thursday evening.