At about 4:40 p.m., an F-Market streetcar struck a 10-Townsend bus, which then hit a private vehicle on Market Street at Second Street.
"Preliminary information indicates that a mechanical failure may have been a factor," police Officer Samson Chan said.
He said that is based on a statement made by the streetcar's operator and a "preliminary investigation looking at the mechanical functions of the brake."
However, Chan said the official cause of the crash is still under investigation.
The streetcar was traveling about 5 mph when it struck the back of the 10-Townsend bus, according to Chan. He said five people were taken to the hospital and seven others were treated at the scene. None of the injuries was considered life threatening.
Muni spokeswoman Kristen Holland said both Muni operators were among those taken to the hospital.
Holland said they will both be tested for drugs and alcohol and then be placed on non-driving status, as is standard procedure.
Bus shuttles provided service on Market Street between the Ferry Building and 11th Street until the accident was cleared at about 6:45 p.m.