The crash happened on Interstate 390 in Steuben County at 4:15 p.m., and a preliminary report indicates that a tire blowout might have caused the wreck, New York State Police spokesman Mark O'Donnell said.
The bus, owned by Bedore Tours of North Tonawanda near Buffalo, left Washington on Sunday morning and stopped for lunch in Pennsylvania, O'Donnell said. He didn't have any details on the passengers.
O'Donnell said investigators will interview the driver and witnesses and inspect the vehicle as they try to determine what caused the crash. The driver lost control of the bus about 55 miles southeast of Rochester and veered down a grassy bank into the woods.
Three patients were taken by helicopter to Strong Memorial Hospital in Rochester and a fourth was taken there by ambulance. Eight were taken to Noyes Memorial Hospital in Dansville. The rest were taken to other hospitals in the region.
The bus owner didn't immediately return a call seeking comment Sunday night.
In January 2005, a bus chartered by a Canadian women's youth hockey team rammed a parked tractor-trailer along I-390 near Geneseo, about 30 miles north of Sunday's crash, killing four people. Investigators determined the driver's lack of sleep and inexperience led to the crash.