The 100-foot-long girder sagged across three lanes of Interstate 70, shearing off the top of an SUV that was carrying a man, a woman and a young girl, said State Patrol Master Trooper Ron Watkins.
All three were killed but no other vehicles were involved in the accident, which occurred shortly after 10 a.m., Watkins said. The site is 10 miles west of downtown Denver.
A caller to radio station KOA described seeing a blue Dodge SUV cut in half.
"The front, from the driver's seat forward, is about 200 feet down the road from the other part that's under the girder," the caller told KOA. The station did not identify the caller.
The girder was put up Tuesday as crews were adding a lane to a bridge carrying Colorado 470 over I-70, said state Department of Transportation spokeswoman Stacy Stegman. The beam was resting on concrete pillars at both ends, with the middle draped across all three lanes of the highway. The crushed remains of the car were underneath.
"This is about as bad as it gets," State Patrol Capt. Jim Wolfinbarger said.
Gov. Bill Owens visited the scene Saturday and offered condolences to the family of the unidentified victims.
"I'd say I'm sorry. This is a horrible tragedy, one that you wouldn't want to have happen to anybody, and we're going to do everything we can to learn from it so it doesn't happen again," he said.
Engineers were on the scene investigating, Stegman said, adding that no vehicles were on the bridge when the accident occurred.
"We can't figure out why on earth it fell," Stegman said.
Asphalt Specialties Co. was overseeing the construction project, Stegman said. The governor said the company was cooperating with state and federal investigators. The phone rang unanswered at the company's Colorado office on Saturday.
Asphalt Specialties is an experienced company with about six other state highway projects currently under way in the state, Stegman said.
Traffic on I-70, the main route from Denver west over the Rocky Mountains, was stopped in the eastbound lanes and moving slowly in the westbound lanes after the accident.