NTSB probing 2-bus Baltimore crash that killed at least 6

There were virtually no skid marks at the crash scene, suggesting that the brakes of the school bus were not forcefully applied, and leading to what Baltimore Police spokesman T.J. Smith called a working theory that the driver had suffered some sort of medical emergency.

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.

Baltimore Police Commissioner Kevin Davis said riders on the No. 10 MTA bus were likely going to work on a route from Dundalk, a largely blue-collar community southeast of Baltimore, toward Catonsville, a western suburb.

Veering across the center line, the yellow school bus slammed into the front driver side of the MTA bus, ending about 100 yards from the pillar.

The school bus raked the side of the commuter bus, ending with its front end buried toward the back of the MTA vehicle on Frederick Avenue near Loudon Park Cemetery.

The school bus was contracted from Baltimore-based AA Affordable Transportation and served 18 elementary students, the city's public school system said in a statement.