Idaho state police said a preliminary investigation showed that the bus driver had a medical condition that caused the crash. The police did not immediately provide more details.
The Salt Lake Tribune said witnesses at the scene identified the person killed as the school's woodwind instructor. Police said the victim's name was not being released pending notification of family.
The crash occurred around 7 p.m. MDT on Interstate 15, about two miles north of McCammon and 50 miles north of the Utah border, as the band headed back to American Fork High School after a competition with several other bands in Pocatello.
James Kimball, 16, a band member at American Fork, told the AP that he saw the bus swerve off the road and go on its side in a ditch off I-15.
"I was on the bus right behind it. We were watching a movie and I looked forward and saw the bus bouncing across the side of the road," the high school junior told the Tribune in a separate interview. "The bus flew off to the side, bumped across a couple hills and rolled over."
He said he saw at least five people being taken away in ambulances. The state police said two students with non-life threatening injuries were transported by helicopter to Portneuf West Medical Center in Pocatello. Other students on board the bus were taken to the hospital to undergo tests.
Distraught students sang church hymns after learning of the instructor's death, Kimball said.
Mike Huestis, the band's assistant director, told the newspaper that authorities worked to ensure that all people on the trip were accounted for and examined for injuries. Police said the bus had about 50 students, two adults and one driver on board.
Huestis traveled to the competition in his own car and was ahead of the two buses. He did not see the crash, he said.
Kimball said the band, one of Utah's best known, won the Pocatello competition. The school's bands have won the Utah state title for the past 19 years. The school band also performed at President George Bush's 2005 inauguration and marched in the 2007 Macy's Thanksgiving Day parade in New York.
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