A collision between an F-line streetcar and a 9L-San Bruno bus this morning has caused a traffic snarl on Mid-Market at 6th Street, and injured at least 19 people. The eastbound streetcar slammed into the back of the bus around 11 a.m., knocking down passengers on the bus including 31 schoolchildren from the Vision Academy day camp.

According to KTVU, four people were taken to the hospital for further treatment, and as the Chron reports, about a dozen children were seen sitting along the sidewalk after the crash with ice packs on their heads.

It's unclear what caused the crash, apart from the F-line operator possibly missing that brake pedal.

For the time being, Market Street is closed to traffic. We'll update you as we learn more.

We have a couple of photos from the scene via Alice Engstrom and Erik Parra.