Eleven people were taken to the hospital after the bus they were on swerved to avoid a tractor and crashed.
Colorado State Patrol says the tractor had running and flashing lights, but did not have the required "slow moving vehicle" sign on the back.
Investigators say the bus driver came up behind the tractor and swerved into the left lane to avoid hitting it. The bus reportedly skidded past the tractor and off the right side of the road where it hit a concrete barrier and tipped onto its side.
Twelve passengers were on board. Eleven were taken to the hospital. Their injuries range from moderate to serious. The driver had minor injuries according to State Patrol.
The bus was on highway 82 on a route between Aspen and Glenwood Springs when the accident occurred at 7:05 p.m. Saturday.
The accident is still under investigation.