Structural deficiencies in the Interstate 35W bridge that collapsed Wednesday were so serious that the Minnesota Department of Transportation last winter considered bolting steel plates to its supports to prevent cracking in fatigued metal, according to documents and interviews with agency officials.

The department went so far as to ask contractors for advice on the best way to approach such a task, which could have been opened for bids later this year.

MnDOT considered the steel plating at the recommendation of consulting engineers who told the agency that there were two ways to keep the bridge safe: Make repairs throughout the 40-year-old steel arched bridge or inspect it closely enough to find flaws that might become cracks and then bolt the steel plating only on those sections.

Fears about bridge safety fueled emotional debate within the agency, according to a construction industry source. But on the I-35W bridge, transportation officials opted against making the repairs.

Officials were concerned that drilling thousands of tiny bolt holes would weaken the bridge. Instead, MnDOT launched an inspection that was interrupted this summer by unrelated work on the bridge's concrete driving surface.

"We chose the inspection route. In May we began inspections," Dan Dorgan, the state's top bridge engineer, said. "We thought we had done all we could, but obviously something went terribly wrong."

Dorgan said there was enough money in the agency's budget to pay for construction work on the underside of the bridge. But he and Gov. Tim Pawlenty acknowledged that transportation officials will face tough questions about the state's upkeep of the bridge, which has had known deficiencies since 1990.

"We will absolutely get to the bottom of this," Pawlenty said. "There were a lot of decisions made, a lot of judgment calls made, and they're all going to have to be critically reviewed."

Pawlenty said an independent consultant will be hired to scrutinize MnDOT inspection practices meant to safeguard the state's 13,026 bridges. In the case of the I-35W bridge, MnDOT inspections convinced officials that the bridge wouldn't need to be replaced or overhauled until 2020, the governor said.

Was there an internal debate?

State and federal officials said it was too early to speculate on what caused the eight-lane bridge to collapse during Wednesday's evening rush hour, but Dorgan said the focus of the investigation is on the bridge's superstructure, or steel underside.

He said inspectors have long been on the lookout for metal fatigue and cracking in the bridge because it was designed before engineers learned about dangers to bridges from fatigue cracking.

"Up until the late 1960s, it was thought that fatigue was not a phenomenon you would see in bridges. Unfortunately that was a wrong assumption," Dorgan said.

According to a source with knowledge of the state and federal investigations, MnDOT is focused on the east side of the northbound section of the bridge past the Washington Avenue entrance as the likely spot where the bridge first gave way.

Bob McFarlin, assistant to the commissioner at MnDOT, dismissed speculation that the collapse was somehow linked to corrosion from de-icing chemicals automatically dispensed on the bridge.

The National Transportation Safety Board is conducting the official investigation and Minnesota has hired its own forensic engineering firm to conduct a parallel study.

A construction industry official who met with MnDOT about shortcomings on the I-35W bridge told the Star Tribune that there have been ongoing concerns among some MnDOT employees about the safety of this and other similar bridges.

"There were people over there that were deathly afraid that this kind of tragedy was going to be visited on us," the industry official said. "There were people in the department that were screaming to have these replaced."MnDOT has been trying to move these 'fracture critical' bridges up in their [budget] sequencing so something like this wouldn't happen," the source said.