The accident happened Saturday evening on a regional route between Aspen and Glenwood Springs, Colo., Colorado State Patrol spokesman Nate Reid said.
The Roaring Fork Transportation Authority bus driver was westbound on Highway 82 when he came up behind a slow-moving tractor. The tractor had on its running and flashing lights, but didn't have a mandatory slow-moving vehicle sign, The Denver Post reported Sunday.
"The driver of the bus attempted to avoid colliding with the rear of the tractor by quickly changing lanes into the left lane. The driver of the bus then lost control and the bus began to skid," the Colorado State Patrol said in a release.
The bus skidded past the tractor, crossed into the right light and hit a concrete barrier on the right shoulder. The bus then rolled over onto its left side, the Post said.
Of the 12 passengers on board, plus the driver, three were seriously injured and eight had moderate injuries. The driver had minor injuries and didn't require treatment at a hospital. Six passengers were taken to Valley View Hospital for treatment and five were taken to Aspen Valley Hospital, the newspaper said.