SAUSALITO -- An off-duty San Francisco police officer reported firing a gun at another motorist in self-defense during a highway confrontation in Marin County early Wednesday, according to the California Highway Patrol.

The incident happened around 12:45 a.m. today on northbound U.S. Highway 101 south of the Waldo Tunnel.

The CHP reported that a passenger in a green Ford pickup truck pointed a handgun in the San Francisco officer's direction, at which point the officer fired his service weapon.

It is unclear whether the Ford or its occupants were struck by the gunfire, CHP Sgt. Diana McDermott said.

The officer, who was not in a police vehicle at the time, and called the CHP immediately afterward, she said. The CHP searched the area but did not locate the Ford pickup.

McDermott said the CHP was still investigating how the confrontation unfolded.

"From what I understand it was a road-rage type of deal that started on the Golden Gate Bridge and proceeded northbound toward the Waldo Tunnel," McDermott said.