The bolt that caused a cable car to slam to a stop Wednesday, injuring seven people, fell off a plate on the tracks and became lodged in the cable groove, officials said Thursday.

The Hyde Street line cable car was heading north on Powell Street just past Washington Street at 10:15 a.m. when it hit the 1 1/2-inch bolt, which had fallen into the channel containing the cable that pulls the car.

Among those hurt were an elderly man whose injuries were considered life-threatening and the cable car’s conductor and grip operator, who suffered a badly bitten tongue and injured ribs, respectively.

Investigators did not know why the bolt fell out of place, Muni spokesman Paul Rose said. Other cable cars passed through the same stretch safely about 10 minutes before the accident, he said.

Muni crews surveyed every similar bolt on the cable car lines after the incident and will conduct weekly inspections from now on, Rose said.