SAN FRANCISCO (AP) — More than a dozen people were hurt when a San Francisco cable car abruptly stopped Monday afternoon.

Fifteen people had injuries ranging from minor to moderate, the San Francisco Fire Department posted on social media. Two of them were immediately taken to hospitals and 11 others were treated for what the department described as “minor aches and pains.”

The San Francisco Municipal Transportation Agency, which runs the cable cars, said it was actively investigating the incident and gave no reason for the stop.

The cable cars are a major tourist draw and were immortalized in a Tony Bennett song about leaving his heart in San Francisco. Passengers on cable cars do not wear seat belts and often hang off the cars, which are partly open-air.

Cable cars first began running in San Francisco in the 1870s and they were designated as a National Historic Landmark in the 1960s. Today there are three cable car lines in the city.

“Safety for our passengers on all Muni vehicles continues to be our top priority. We’ll be conducting a full review of incident details to ensure continued safety on the cable cars,” SFMTA said in a statement.