A motorcyclist who was not wearing a helmet suffered life-threatening injuries when he flew off the back of the motorcycle in a hit-and-run crash in Boston early Sunday morning, according to Massachusetts State Police.

The motorcycle passenger, a 22-year-old Boston man, is in critical condition at Beth Israel Deaconess Medical Center.

The motorcycle’s operator — a 24-year-old Boston man who was also not wearing a helmet and was ejected during the crash — was transported to Brigham and Women’s Hospital with non-life threatening injuries.

Massachusetts State Police troopers responded to the hit-and-run crash on the Arborway at South Street in Boston, just before 2 a.m. on Sunday. The crash involved a 2003 Honda Accord and a 2016 Honda CB300 motorcycle.

The preliminary investigation by the Massachusetts State Police Collision Analysis and Reconstruction unit shows that the motorcycle was eastbound on the Arborway.

Then at the intersection with South Street, the Honda Accord — which had been westbound on the Arborway approaching the intersection — entered into an illegal left turn from the Arborway to South Street, police said. The car and motorcycle collided in an eastbound lane of the Arborway.

After the crash, the driver of the Accord did not stop and drove away from the scene, heading south. Troopers were working to find that driver and the vehicle.

The ongoing investigation into the crash is being conducted by the State Police-Milton Barracks, the State Police Collision Analysis and Reconstruction Section, the State Police Crime Scene Services Section, and the State Police Detective Unit for Suffolk County.