LYONS, Colo. (AP) _ Brake failure may have caused a school bus to plunge off a mountain road, killing a sixth-grader and injuring 36 other schoolchildren, officials said.

Forty-one people, including four adults and 37 schoolchildren from Whittier Elementary School in Boulder, were aboard the bus Friday as it went off the road, plunged about 20 feet down an embankment and overturned onto rocks three miles southwest of Lyons in central Colorado.

''Everyone was screaming, the bus was tipping ... we were going too fast,'' said Marc Tedesco, a sixth-grader on the bus. ''We were heading towards the rock and she turned to miss it and we did a 360.''

Kevin Swenby, 11, was reported dead at the scene. Kristopher Sieger, 12, was listed in serious condition at St. Anthony's Central Hospital in Denver Friday night, according to a hospital spokeswoman. Marissa Burns, 11, was admitted to Longmont United Hospital, where she was in stable condition.

The rest of the passengers were treated at area hospitals and released, officials said.

Officials said the brakes may have failed as the driver, 41-year-old Mireya Van Amee, tried to turn on a curve on the steep mountain canyon road.

Skid marks left along the canyon road by the bus indicate that it slid out of control for at least 1,000 feet before it went over the embankment, said Mark Anello, a Colorado State Patrol spokesman. ''It looks like a brake problem,'' he said.

The children were returning from a field trip at Highlands Presbyterian Church Camp, according to Lynn Straight, Boulder Valley Public Schools associate superintendent.