SAN FRANCISCO >> Seven people were injured Wednesday when a San Francisco cable car carrying about 40 passengers came to a sudden stop in the city’s Nob Hill neighborhood, authorities said.

One of the injuries to an elderly man who fell and hit his head was considered potentially life-threatening, San Francisco fire spokeswoman Mindy Talmadge told The Associated Press.

Four other people were taken to a hospital with non-life-threatening injuries.

It appeared a 1.5-inch bolt stuck in the cable car’s track caused the sudden stop, San Francisco Municipal Transportation Agency Director John Haley told reporters at the scene.

Officials believe the bolt did not belong to other cable cars that travel the route. Two other cable cars that passed safely through the site about 10 minutes before the accident were being inspected.

San Francisco Police Officer Mary Godfrey told KTVU-TV that a female passenger suffered a leg injury when she was thrown from the cable car.

The car’s two operators were among the injured, according to an official with the union that represents San Francisco transit workers.

The conductor underwent surgery after suffering facial and tongue injuries, while the gripman had internal injuries and cracked ribs, said Ronald Austin of the Transport Workers Union of America Local 250-A.

Service resumed shortly before 2 p.m.