Workers fell behind schedule because the girder was first installed backward and had to be turned around, investigators said.
The collapsed girder had been secured with temporary braces that were not part of the original job order but were devised by the crew foreman and approved by the state transportation department, National Transportation Safety Board investigator Ken Suydam said.
Four of the five braces were found bent after the collapse Saturday onto Interstate 70. The ends of the 100-foot girder were not fastened to the bridge, to which a lane was being added.
The work began May 11, but bad weather kept workers from installing a second girder that would have helped shore up the beam already installed, NTSB investigator Dan Walsh said.
The girder collapsed Saturday morning, shearing in half the SUV carrying William J. Post, 34, wife Anita M. Post, 36, and their 2-year-old daughter, Koby Anne Post, all of suburban Evergreen.
The girder was manufactured in two sections, which were joined at the work site about 10 miles west of downtown Denver.
One of the sections was incorrectly marked at the foundry, which led the workers to install it backward, said Dave Minshall, spokesman for Ridge Erection Co., the subcontractor that did the work.
Minshall said the temporary fix was a "decision that was made at lots of levels." Transportation Department spokeswoman Stacey Stegman said there was no documentation that the department signed off on the temporary solution.
The NTSB is trying to set up interviews with the construction workers and plans to review the diary kept by the state inspector on the scene.
Investigators are also looking at whether vibration from the road or changes in temperature caused the girder to shift and fall. The girder was installed on a bridge carrying state highway C-470 over I-70.
The State Patrol has said one of its dispatchers received a call from a driver an hour before the accident warning that the girder looked like it had rolled two or three feet. The dispatcher told a state transportation crew to look for a downed sign instead.
"Everyone at CDOT feels terrible as do the contractor and subcontractor on the project," the Transportation Department said in a statement.