SAN FRANCISCO -- A runaway tow truck that had been unloading a car onto a steep, rain-soaked street damaged 10 parked cars and temporarily shut down San Francisco Municipal Railway service Monday morning in the city's Inner Sunset neighborhood.

The incident took place at Third Avenue between Irving Street and Parnassus Avenue at about 10:30 a.m., police spokesman Officer Samson Chan said.

According to Chan, the flatbed tow truck had just begun unloading a woman's car when both vehicles began sliding down the street.

At some point, the car became separated from the truck and hit a tree, while the truck continued downhill, causing major damage to several of the parked vehicles, Chan said.

The woman was inside her car when the incident occurred, but jumped out before her car hit the tree, Chan said. She sustained minor injuries.

A man inside one of the parked cars that was struck by the tow truck also suffered minor injuries.

The crash halted Muni service on the N-Judah light-rail line for about two hours, according to Muni spokesman Judson True.

The driver of the tow truck cooperated with police in the investigation, according to Chan.

"It's still a very open and active investigation, and we're looking at whether it was mechanical failure or operator error," Chan said.