BRIDPORT, Vt. — A Vermont State Police investigation has been unable to determine why a car crossed the center line of a Vermont highway last summer and slammed into a pickup truck, killing the car’s driver and three passengers.

But police said Friday that a toxicology report found that the driver had high levels of marijuana in his system and that may have contributed to the Aug. 7 crash on Route 22A in Bridport. They say speed and fatigue may also have been contributing factors.

Police say the toxicology report found the 20-year-old driver had a high level in his system of Delta-9 THC, the psychoactive ingredient in marijuana.

The driver and passengers were not wearing seat belts.

The two people in the other vehicle suffered minor injuries.