SAN FRANCISCO—An alarm that alerts authorities about possible obstructions on San Francisco's world-famous cable car line didn't sound off before a cable car hit a bolt and came to an abrupt stop, injuring seven people, transit officials said Thursday.

The alarm didn't sound because the loose bolt came from a street surface plate that was lodged into the tracks, San Francisco Municipal Transportation Agency spokesman Paul Rose told The Associated Press Thursday. The bolt was above the underground cables that has sensors around it, he said.

Wednesday's mishap occurred after a cable car with about 40 people onboard going downhill at around 9 miles per hour struck a 1 1/2 inch bolt in the cable groove and came to a sudden halt. Two other cable cars had passed safely through the site about 10 minutes before the accident.

The agency now plans to conduct weekly checks on each of the bolts and the panels along the renowned five-mile cable car route "to make sure they are secure as possible," Rose said.

Transit director John Haley said Wednesday that several items including those as light as pieces of paper in the past have set off the system's alarms, so it's unclear why the metal object didn't.

Among the seven injured included a conductor who underwent surgery Wednesday from facial and tongue injuries and a gripman who suffered internal injuries and cracked ribs, transit officials said.

Five were taken to a hospital, including an

elderly man in his 80s with potential life-threatening injuries he suffered a head injury, authorities said. His condition was not immediately known.