GOLDEN, Colo. -- Engineers are trying to determine what caused a huge steel girder to fall across a highway near Denver Saturday, killing three people.

The Colorado State Patrol said the 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.

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All three were killed.

No other vehicles were involved in the accident, which was 10 miles west of downtown Denver.

Casey Stegman, a spokeswoman for CDOT. She said the girder weighed several tons and was just put in place on Tuesday by crews widening a ramp.

"We have no reason to believe that this should have happened," Stegman said. "We are looking into it. It's very tragic."

Structural engineers were called to the scene of the accident to inspect the bridge.

Gov. Bill Owens will also inspect the damage later Saturday afternoon.

The girder split the van in half, with the front passenger section coming to rest several hundred yards beyond the back section. Agencies from several jurisdictions rushed to the scene when the accident was first reported as a bridge collapse.

No cars were on the bridge at the time the girder fell because the bridge was under construction.

The accident backed up traffic on I-70 in both directions for miles.

Witnesses say a large crane was working in the area when they heard a large boom just after 10 a.m.