A school bus blocks away from its first stop Tuesday morning rear-ended a car, hit the entrance to a cemetery and then veered into an oncoming commuter bus in a crash that killed at least six people and injured 10, authorities said.

No children were on board the school bus, whose driver was killed along with at least five people on the Maryland Transit Administration bus, Baltimore Police spokesman T.J. Smith said.

“It literally looks like a bomb exploded in the bus. It’s catastrophic damage,” Smith said.

The only other occupant of the school bus, an aide, was taken to a hospital, as were the car driver and eight people from the commuter bus, Smith told a news conference. Their injuries ranged from minor to critical, he said.

The school bus was on its way to pick up a child a few blocks away, CBS Baltimore reported.

The school bus slammed into the MTA bus, crumpling both driver’s areas, and then raked the side of the commuter bus, tearing off sheet metal. It finally came to a stop with its front end buried toward the back of the MTA bus.

Smith noted a lack of skid marks at the crash scene on Frederick Avenue near Loudon Park Cemetery, leading to what he called a working theory that the school bus driver had suffered a medical emergency.

CBS Baltimore spoke with a witness who came upon the crash shortly after it happened. He described trying to free an injured woman before emergency workers arrived on the scene, saying there was glass all over her. He and another man lifted the front windshield off of her but they could not get her out of the bus.

Firefighters were still working their way through the wreckage of the commuter bus about two hours after the 7 a.m. crash, Baltimore Fire Chief Niles Ford said.

“This was a significant, significant wreck, so there are still portions of the bus that our people have not been able to fully access,” Ford said.

He said firefighters also had to enter the school bus from the rear and cut out the seats to reach the driver and aide.

Smith characterized it as an accident investigation, not a crime-scene investigation, despite the presence of homicide detectives. Smith said they were called because they are accustomed to conducting death investigations.

University of Maryland Medical Center spokeswoman Lisa Clough said the hospital received one patient in critical condition and four others in fair condition.

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