— The family of a North Carolina State University campus bus driver who died in December after complaining of fumes on her bus said Thursday that an autopsy found the cause of her death as carbon monoxide poisoning.

Joann Veronica Griggs Sewell, 50, died three days after her family said she was overcome by fumes on a Wolfline bus on N.C. State's Centennial Campus.

Patricia Ray, the union representative for Teamsters Local 391, which represents First Transit's Wolfline drivers, said two other drivers complained about fumes on Sewell's bus earlier that day.

First Transit, which has operated the Wolfline system since 2007, took the bus out of operation while the North Carolina Department of Labor investigated her death.

Sewell's family requested an autopsy following her death, and daughter Sherese Brown said the results indicate her mother died of carbon monoxide poisoning.

The State Medical Examiner's Office said Sewell's autopsy isn't complete and that preliminary results aren't released publicly, so officials couldn't confirm the family's information.

Cincinnati-based First Transit issued a statement Thursday afternoon, noting that tests of air on the Wolfline buses haven't found detectable levels of carbon monoxide. The company said it continues to cooperate in the investigation into Sewell's death.

"Our maintenance staff routinely evaluates the fleet to ensure safe and efficient services," the statement said. "A recent audit of the fleet did not reveal any material issues – be it fumes on buses or any other safety related issues."

Last month, another Wolfline driver also complained that smells on her bus made her sick.

Lisa Feeney said some of her passengers also noticed the fumes, but First Transit maintenance workers found nothing wrong.

Feeney said she refused to drive the bus after that, but the bus continued on its route with a different driver.