Seven people were injured Wednesday, one seriously, when a cable car in which they were riding hit a small object lodged in the line and slammed to an abrupt stop on Nob Hill, authorities said.

Five people were taken to a hospital, one of them with life-threatening injuries, after the 10:15 a.m. accident on Powell Street near Washington Street, said Lisa Filiss, a spokeswoman for the Fire Department. Two other people who were injured declined medical treatment.

The badly injured passenger is a man in his late 80s who hit his head, said Mindy Talmadge, another Fire Department spokeswoman. She did not know whether the man had struck the ground or something else.

The cable car, with 40 people aboard, was traveling north at about 9 mph downhill on Powell and had just passed Washington when a metal object in the cable groove - a 1 1/2-inch screw or bolt - caused the car to lurch to a stop, said John Haley, director of transit for the Municipal Transportation Agency.

The object "does not appear to have come from a cable car, but we're still looking into it," Haley said.

The cable system has alarms that are supposed to alert authorities of obstructions, but none sounded in this case, Haley said. In the past, pieces of paper have set off alarms, so it was unclear why the metal object did not do so, he said.

Another cable car had safely passed over the spot about 10 minutes before the accident, Haley said.

Both the conductor and grip operator were among those who were hurt, said officials with the Transport Workers Union. The conductor severely bit his tongue and had a concussion, while the grip operator suffered rib and hand injuries when he fell onto the grip.

Cable-car service on Powell Street resumed about 1:45 p.m.