By Derrick Blakley

CHICAGO (CBS) — Union officials believe it was a medical incident–not a mechanical fault or human error–that was behind this week’s deadly CTA rush hour bus crash.

A representative with the bus driver’s union thinks the driver may have suffered a seizure.

Two days after the crash at Lake Street and Michigan Avenue, which killed pedestrian Aimee Coath, driver Donald Barnes remains hospitalized at Northwestern, recovering from serious head injuries.

“I don’t even know if he knows there was a fatality yet,” said Carlos Acevedo, a representative of Barnes’ union.

After initially stopping at the red light, the No. 148 bus turned left from eastbound Lake to northbound Michigan, plowing into several cars and onto the sidewalk.

Union officials say they are certain that the driver was wearing a seatbelt.

“If he didn’t, he would have been laying unfortunately on the street because you see how the windshield came out,” Acevedo said. “Our operators are really good about putting on their seatbelts.”

The fact that the bus stopped properly before proceeding leads union officials to suspect Barnes blacked out or had a seizure. Both possibilities are being explored during hospital tests.

“They’ve got him down there with the police department, and they’re running a barrage of tests on him to check everything out,” Acevedo said.

Operator fatigue was a factor in last year’s blue line rail crash at O’Hare.

The transit union doesn’t think that played a role here. Barnes worked a split shift but should have been rested.

“If there’s anything to be learned as far as safety, I know us and the CTA will work together to make it happen,” Acevedo said.

Sources told CBS 2 that Barnes has no history of blackouts or seizures, but he does suffer from high blood pressure.

If Barnes did have a seizure, he would have to go for one year without another one before being allowed to drive a bus again.