Police say ‘driver fatigue’ may have been a contributing factor in a collision between two tractor trailers that shut down a portion of Interstate 80 early Tuesday morning.

It happened on the westbound lanes of I-80 at mile marker 361, outside Cheyenne, Wyoming.

According to the Wyoming Department of Transportation, a Swift Transportation tractor trailer was driving west on I-80, in foggy conditions, when it struck a KLLM truck that was parked on the shoulder of the highway.

Sgt. Kyle McKay of the Wyoming Highway Patrol says the Swift truck drifted into the emergency lane and struck the KLLM truck from behind, causing significant damage to both vehicles.

Sgt. Mckay told reporters that driver fatigue may have been a factor in the crash. Police are also looking into why the KLLM truck was stopped on the shoulder.

Both drivers, as well as a passenger , were taken to nearby hospitals for evaluation. The injuries are not believed to be serious, according to the Wyoming Tribune Eagle.

The highway was closed for more than 5 hours, but has since reopened.