Gunman on the run after 2 killed in Houston highway shooting

A gunman and his accomplice are still on the run after killing two people on a major highway in Texas during rush hour Thursday evening, authorities said.

Houston Police are now offering a reward of up to $5,000 for information that leads to the arrest of the suspect in the attack.

The deadly shooting occurred just before 6 p.m. local time when the suspect's car struck another vehicle on Interstate 10 in Houston, causing it to spin out similar to a PIT maneuver. The suspect and another individual in his car got out in the middle of the freeway.

Then the suspect, believed to be armed with an assault-style rifle, opened fire at the other vehicle, according to officials with the Houston Police Department.

The gunman followed the car as it rolled backwards down the freeway, firing multiple rounds. He continued to shoot at the windshield even after the vehicle came to a stop, police said.

The vehicle's occupants -- two men who were 25 and 33 years old -- were killed. Narcotics were later found inside their car, police said.

A bystander who saw the shooting took out his own gun and fired several rounds at the suspect, who then ran back to his car, a dark-colored Sedan, and fled the scene, police said.

It's unknown if the suspect was hit.

Police described the suspect as a man in his 20s who was wearing a red shirt.

The section of the highway where the shooting occurred remained closed overnight while investigators processed the crime scene.

The investigation is ongoing and police are considering all possible motives. Police are looking for both men from the suspect's vehicle.

"Road rage? An accident? Narcotics? We don't know yet," Houston Police Assistant Chief W.R. Dobbins told reporters at a press conference Thursday night.

ABC News' Ella Torres contributed to this report.