MINNEAPOLIS — Trucks, loaders, and paving equipment rumbled onto the Interstate 35W bridge in mid-June as Progressive Contractors Inc. began a $2.4 million project to repair sections of the heavily traveled highway.

As tens of thousands of cars zoomed by every day, Progressive’s workers pounded away at the road surface with jackhammers, cut loose pavement with industrial-strength saws, and mixed and poured concrete.

The bridge collapsed Wednesday toward the end of a shift, taking Progressive’s 18-person crew with it. One worker is feared dead.

For now, the construction project is only one item on a long list of possible causes that also includes aging steel, rotting welds, vibrations from adjacent train tracks, and even the corrosive effects of bird droppings. But the company’s work on the structure has become a crucial early part of the investigation by the National Transportation Safety Board.

Yesterday, the NTSB said it had interviewed company officials and workers and was analyzing construction and maintenance documents.

Employees of Progressive, based in St. Michael, Minn., have helped investigators map out the locations of its equipment, vehicles, and materials at the time of the accident, and how much each piece weighed.

NTSB chairman Mark Rosenker called the interviews “quite informative” but declined yesterday to say what they showed.

A helicopter with a camera similar to those used in Hollywood movies will peer into the wreckage. Laser-guided surveying equipment has helped produce an in-depth map of the debris. Software re-creating the disaster on a computer screen may even be able to pinpoint the exact piece of bridge that gave way.

“Computers and modeling techniques are just light years from what was available 40 years ago,” said Ted Galambos, a professor emeritus of structural engineering at the University of Minnesota and an expert in the stability of structural steel. “Now we can have an idea, and we can test that on a computer in a few hours.”

The dive recovery teams are also turning to technology for help, using underwater video cameras to look under dangerously unstable debris. Local teams have also requested help from FBI and Navy dive teams in the search for the eight people feared dead in addition to the five confirmed victims. State transportation officials said they would begin removing bridge debris from the Mississippi River later this week. One goal of clearing the wreckage is to open a channel at least 56 feet wide to accommodate barge and boat traffic. Officials offered no timetable for how long it would take.

Investigators caution that it could take as long as 18 months to complete their exhaustive probe into why Minnesota’s busiest bridge collapsed and fell into the river Wednesday.

But they already have begun zeroing in on clues.

On Friday, they were focusing on the south section of the bridge, where they quickly found that the span shifted 81 feet during the collapse.

On Saturday, the north side became the focus. That’s where they plan to use a helicopter equipped with a high-resolution camera that can examine the debris in precise detail for any troublesome signs. The camera is kept steady by a gyroscope — which is how Hollywood crews get smooth footage while filming from a vibrating helicopter.

Investigators also plan to watch frame-by-frame enhancements of video of the collapsing bridge. In addition, the FBI used laser-guided surveying equipment to complete a detailed 3-D map of the wreckage, and quickly provided the data to the lead investigation agency, the National Transportation Safety Board.

Nineteen NTSB investigators from around the country are in Minneapolis, working out of trailers, hotels, and command posts.