At least six people were killed when a school bus hit a car, careened into a pillar and struck a commuter bus in southern Baltimore on Tuesday morning, according to local police.

The bus was on its way to pick up a student, police say — there were no children on the bus at the time of the accident.

The series of crashes began when the yellow school bus struck the back of a Mustang. The driver of that car was injured, according to law enforcement officials.

The school bus, which had the driver and one aide on board, then struck a pillar at a cemetery. It veered into oncoming traffic, where it crashed into the Maryland Transit Administration bus nearly head-on, striking the driver's side and ripping open the side of the bus.

More than a dozen people were traveling on the commuter bus at the time.

"Something catastrophic took place," Baltimore police spokesman T.J. Smith said. "It literally looked like a bomb" had gone off inside the commuter bus.

There were no skid marks to indicate that the school bus had attempted to brake, Smith said. The cause of the crash is under investigation, he said.

The driver of the school bus was killed in the crash, as were five people on the Maryland Transit Administration bus.

At least 10 people were injured, including the aide who was traveling on the school bus and a number of passengers on the commuter bus. Injuries range from minor to critical, police say.

Police say they hope to know more when they interview the aide.

Images from the scene show the fronts of both buses badly crumpled, with the driver's side of the commuter bus ripped open by the front of the school bus.

The Baltimore Sun reports that the school bus in the accident was contracted for the city school system, but could not verify which school it was associated with.

Copyright 2021 NPR. To see more, visit https://www.npr.org.