SAN FRANCISCO » Police on Saturday were investigating the possibility of gang involvement in the killings of four men shot to death in a trendy San Francisco neighborhood.

The four were shot while sitting in a double-parked car around 10 p.m. Friday in the Hayes Valley district, and died at the scene, police spokesman Albie Esparza said. Police on Saturday were trying to determine the make and model of a car that witnesses saw speeding from the scene, and to determine if it was linked to the killings, Esparza said in a statement.

Police Chief Greg Suhr told the San Francisco Chronicle that police gang task-force and homicide units were combining efforts in case the killings could trigger any retaliatory violence.

"We need to figure out why this happened. If we can discover that, then we will know if there is another shoe that is going to fall," Suhr said. "We need to be out in front of that."

Residents of Hayes Valley, an area that includes popular restaurants, shops and art galleries, called police emergency lines Friday night to report hearing at least 15 shots, the Chronicle reported. San Francisco broadcaster KGO reported some bullets struck inside surrounding homes, including one that lodged in a refrigerator.

The car in which the men were found had been reported stolen, police said.

By early Saturday, police had covered parts of the shooting scene with yellow tarps. Police cordoned off several blocks, and canvassed the area for witnesses.

Police alerted the California Highway Patrol that a vehicle possibly linked to the violence could be headed toward the Pacific Coast Highway.