A school bus with no students on it smashed into an oncoming Baltimore commuter bus during Tuesday's morning rush hour, killing at least six people and injuring 10, police said.

The school bus rear-ended a car, struck a cemetery entrance pillar and then crashed into the Maryland Transit Administration bus, ripping off much of its driver's side, police spokesman T.J. Smith said at a news conference.

"It literally looks like a bomb exploded in the bus, and it's catastrophic damage," he said at the crash scene in southwest Baltimore.

The cause is under investigation. The National Transportation Safety Board said it was sending an investigative team to Baltimore.

Five people aboard the commuter bus, as well as the school bus driver, were killed. School bus seats had to be cut out to reach victims, and the death toll could rise as the wreckage is searched, he said.

Ten people were taken to hospitals with injuries ranging from minor to critical, Smith said.

The school bus was on its way to pick up a student with the driver and an aide aboard. Thirteen people were on the Maryland Transit Administration bus, he said.

There were no signs of skid marks and the accident scene extended over several hundred yards, police said. The Baltimore City School District and the transit agency did not have an immediate comment on the crash.