TRINIDAD, Colo. - A Des Moines doctor and accomplished endurance bicyclist was killed during the Race Across America when he was struck by a pickup truck.

Robert Breedlove, 53, was killed about 10:15 a.m. MDT on Thursday on a narrow two-lane stretch of Highway 12 when he "apparently moved over into the oncoming traffic lane," race director Jim Pitre told The Des Moines Register.

"Nobody knows how or why he ran into an oncoming pickup head-on," Pitre said.

He said the truck's driver told police that Breedlove "appeared to slump down on his handlebars" before moving into traffic.

"It doesn't make any sense to me that a rider with that level of experience would drift into the other side," Pitre said.

Authorities were investigating the incident, which happened 28 miles west of Trinidad in southern Colorado.

June Torres, of the Colorado State Patrol, said there was a lot of traffic on the road at the time of the accident.

Breedlove, an orthopedic surgeon, is an accomplished cyclist known for his endurance feats including riding the weeklong RAGBRAI - Register's Annual Bike Ride Across Iowa - route in one day. In 2002, he set a trans-America solo bicycle crossing record for a person older than 50 by riding from Los Angeles to Atlantic City, N.J., in nine days, 19 hours and 47 minutes.

This race was his fifth solo ride across the United States. In 1994, he finished second in the race, the longest bicycle race in the world.

Breedlove and his wife, Gretchen, have four children.