The driver of a trash truck who authorities said went around the barrier at a railroad crossing and into the path of an Amtrak train carrying Republican lawmakers to a retreat had marijuana and a prescription drug in his system, the National Transportation Safety Board said this week.

The NTSB has not released its final report on January 2018 crash near Crozet, Va., but it posted a probable cause report on the agency’s website that said ​it was “the truck driver’s decision to enter an active grade crossing and his inaction when he encountered obstacles while attempting to cross the railroad tracks, most likely due to his impairment from the combined effects of the drugs marijuana and gabapentin.”

Gabapentin is a prescription drug used to treat mood an anxiety disorders.

A grand jury last year indicted the driver, Dana W. Naylor Jr., on one count of involuntary manslaughter and one count of maiming another while driving under the influence, A co-worker of Naylor’s who was riding in the truck was killed and several lawmakers were injured in the crash. A jury last month found Naylor not guilty of involuntary manslaughter.

One congressman aboard the train suffered a concussion, while several others had minor injuries. The train was carrying the Republicans to a retreat at the Greenbrier Resort in White Sulphur Springs, W.Va.

According to a preliminary report by the NTSB, the truck appeared to have bypassed the crossing gates and was on the tracks when it was hit by the train.

The Amtrak engineer hit the emergency brake, but the train was going too fast to avoid the collision and struck the left rear of the trash truck. Two passengers were ejected from the truck, which came apart, leaving a field of debris.

Video from the train’s front-facing video camera showed that the gates were down as the crossing came into view and that the truck was on the tracks. Witnesses told investigators that the truck entered the crossing after the gates went down.

Lori Aratani contributed to this report.