Traffic investigators say they are confused by a one-car, low-speed accident in downtown Winslow that left two passengers critically injured Thursday.

Police and fire units were dispatched at about 6 p.m. Jan. 1 to lower Madison Avenue near Parfitt Way, where a 1978 Volvo 245 station wagon reportedly left the road and ran head-on into a tree.

Witnesses told police the vehicle was traveling south at a “normal” speed when it veered left across the roadway and went into a vacant lot just past the Windermere building.

Two passengers, Khanh Q. Tran, age 17, and Nguyet T. Tran, age 51, suffered critical internal injuries and were airlifted to Harborview Medical Center.

The driver, San Q. Tran, age 20, was taken by ambulance to the hospital, where he was admitted for observation.

The driver and older passenger were wearing seatbelts, Bainbridge Police Traffic Officer Rob Corn said.

Why the driver lost control of the vehicle was unclear.

“The driver said the car had just ‘stopped,’” Corn said. “The reason that happened, we can’t figure out.

“We kind of went back and forth, and pretty much determined that the engine shut off, and he couldn’t steer or brake.”

Alcohol did not appear to be a factor in the accident, Corn said, and no citations have been issued.

The family is new to the community, the driver told police, having moved to Bainbridge Island about nine months ago.

The vehicle was totaled; the street was closed for more than three hours for the crash investigation.

The rest of the holiday was quiet, police said, with Corn reporting just one drunk-driving arrest.