THORNTON, Colo. (CBS4)– What led to the death of the driver in a deadly bus crash remains a mystery. The coroner’s autopsy report on Kari Chopper doesn’t answer a critical question about the investigation.

Kari Chopper (credit: John Chopper)

Kari Chopper (credit: John Chopper)

READ MORE: CDC Estimates Pfizer Vaccine For Ages 5-11 In Colorado On November 1

The coroner’s report states that Chopper died of injuries suffered when the bus struck a concrete pillar. The coroner was not able to determine whether she had a medical issue that led to the crash.

The coroner previously had told Kari’s husband that she had an enlarged heart and may have had a heart attack.

(credit: CBS)

(credit: CBS)

READ MORE: Douglas County Parents, Students Line Up At School Board Meeting Following Mask Mandate Ruling

The Legacy High School Varsity and JV football teams were leaving DIA on buses after returning from a game in California on Sept. 11. Chopper somehow crashed into a concrete bridge in the passenger pick up area and was killed. Fifteen students were taken to the hospital. Three coaches were seriously injured.

(credit: Denver Police Department)

(credit: Denver Police Department)

Bus drivers are required to undergo a Department of Transportation medical exam every two years. Kari passed a physical exam in May. She worked for the Adams 12 School District for four years and her husband said she loved her job.

MORE NEWS: Denver City Council To Hear Proposal For Lowering Neighborhood Street Speeds

LINK: Kari Chopper GoFundMe page