A crash that killed two people Thursday on I-85 northbound in Gwinnett County happened when the driver of a car swerved into the path of a tractor-trailer, police said.

Two rear-seat passengers of the car died at the scene, police spokeswoman Cpl. Michele Pihera said Friday. Two other passengers, one in the front passenger seat and one in the rear, were injured. The car’s driver, a man in his late 40s, was not injured.

The driver of the tractor-trailer, identified as Shawn Padgett, 34, of Simpsonville, S.C., also was injured, Pihera said.

The identities of the people killed have not been released, pending notification of family members, Pihera said. None of the occupants were children.

Police don’t know why the driver, who was in the Peach Pass lane about 2:19 p.m., lost control of the car, Pihera said. The tractor-trailer was three lanes to the car’s right.

Speed and alcohol do not appear to be factors, Pihera said. Investigators will be searching for defects in the car that might have contributed to the crash.

The wreck, which happened just north of Pleasant Hill Road, shut down the northbound lanes of I-85 for more than two hours. Traffic was diverted onto Highway 316 via an exit while investigators cleared the crash scene.

The tractor-trailer was carrying tens of thousands of tons of steel, Channel 2 Action News reported.

The three people injured in the crash were taken to Gwinnett Medical Center in Lawrenceville.