Sgt. Mike Farr says the bus was going 30 to 40 miles per hour – normal speed for that lane is about half that. The bus was carrying the Legacy High football team from Broomfield.
The driver was killed and 15 students and 3 coaches were injured.
Farr said investigators are checking to see if the driver, a woman in her 40s, had a medical condition, or if the bus had a mechanical problem.
"What were left with now is trying to explain why this crash occurred," Farr said. "Quite frankly there is a big mystery there that we are in the process of attempting to answer. And we may not get all the answers we are looking for there."
Investigators want to know why the driver circled back to DIA after picking up the team.
"A lot of folks thought, 'Maybe we forgot something, forgot a student, forgot some equipment.' But no one could tell me certainly why the bus driver made that turn," Farr said.
Most of the students have been released from the hospital. Three coaches sustained significant injuries and are still hospitalized.