Nearly two months after Treat Williams died in a motorcycle crash, the investigation has taken a surprising turn, as police charged the other driver with negligence.

Ryan Koss surrendered to Vermont State Police Tuesday night and was issued a citation, charging him with grossly negligent operation with death for swerving into the path of the beloved actor.

Koss also posed for a mug shot and was given a September court date for his arraignment before the judge. If convicted, Koss faces up to 15 years in prison and/or a $15,000 fine.

What’s more … the New York medical examiner revealed Treat’s cause of death, which was severe trauma and blood loss.

As we reported in June, Treat was riding his chopper in Dorset, Vermont, and made a left turn into a parking lot, colliding with a Honda Element driven by Koss.

The impact sent Treat flying off his motorcycle, causing him to suffer critical injuries, even though he was wearing his helmet.

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Treat was airlifted to New York’s Albany Medical Center, where he was pronounced dead.

Koss was treated for minor wounds at the scene.