Part of overpass collapses, killing 3 in Colorado
Highway department investigating cause
 |  Part of the overpass has collapsed, touching the interstate below Colorado 470. |
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 A 40-ton steel support girder on Colorado 470 collapses onto Interstate 70 just south of Golden.
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(CNN) -- A 40-ton steel support girder on an overpass west of Denver, Colorado, collapsed onto Interstate 70 on Saturday, killing three people and blocking eastbound traffic on the interstate, a state official said.
The interstate remained closed just south of Golden on Saturday afternoon. The girder fell from an overpass of Colorado 470 that had been under construction.
An investigation into the collapse is under way and will keep I-70 eastbound closed for "at least five or six hours or into tomorrow morning," Colorado Gov. Bill Owens told reporters.
"It's too early to say when the girder can be removed," Owens said.
Stacey Stegman, public information officer for the Colorado Department of Transportation, said the girder had been raised Tuesday night.
The girder fell onto a Dodge Durango traveling east on I-70, "slicing the vehicle into pieces" and killing all three occupants, Stegman said.
The overpass, 16 miles west of Denver in Jefferson County, is part of an interchange to which exit and entrance ramps are being added.
"So far we have no idea what caused the collapse, whether it was the result of some sort of collision with a vehicle or not, but it is under investigation," Stegman said.
