It's a puzzle with twisted, mangled pieces, some jutting high into the air, others submerged under an eddying Mississippi River.
To piece together exactly what caused last week's bridge collapse here that killed at least five people, investigators are turning to a new 3-D laser camera technology and enhancement of surveillance videos.
Starting Monday, a computer simulator will model failure after failure, as investigators hope to find a wreck scenario similar to the scene that is bringing thousands of Minnesotans to the riverbanks to quietly reflect and mourn.
Early clues are leading some outside analysts to cast initial suspicions on recent construction activity on the bridge, wear-and-tear cracks in the metal, or both. And politicians across the nation, responding to sudden safety doubts among the public, are ordering inspections of thousands of bridges.
But federal investigators stressed that they may not have definitive answers, or new safety recommendations, for months.
"This is a long, very long, but thorough procedure," said Mark Rosenker, head of the National Transportation Safety Board (NTSB). "To try to give a bit of perspective on this, we just completed within about a year the final report on the Big Dig [ceiling panel collapse in a Boston tunnel]."
Mr. Rosenker reported significant progress mapping the wreckage, an important step given the river currents and recovery operations altering the scene.
With eight people still believed to be missing, officials said divers are inspecting each submerged vehicle, braving murky waters that supply only a foot of visibility and the glacier-like creaks and moans of the destroyed bridge.
On a tour Saturday of the scene, President Bush lauded the bravery of those involved in rescue efforts: "We have an amazing country, where people's instinct, first instinct, is to help save life."
Throughout the weekend, residents trekked alongside a police perimeter and perched on grassy knolls near the defunct flour mills at the river's edge. Some brought flowers to cast; others, frustration, carrying "build bridges not bombs" placards.
"You can't put up infrastructure on the cheap, and if you are going to do that, something has got to give," says Eileen Johnson, who sells flowers in the city, echoing a common sentiment.
In its quest to pinpoint just what happened, and why, the NTSB got a boost from a 3-D laser scanner brought in by the FBI. The device sits on a tripod and shoots thousands of lasers to take a three-dimensional picture, replacing the tedious work once done one point at a time by surveyors.
"Because you have this mess of twisted metals, it would take you months to do point-by-point maps. This will allow us to do it in a day," says Mark Bagnard, an NTSB investigator.
Officials used the same sort of device after freeway collapse in Oakland, Calif. this past April to get highly accurate measurements of the road gap to be retrofitted.
Municipalities are buying these scanners with the help of Homeland Security funds to produce maps of potential soft targets, says Tony Grissim with Leica Geosystems. The Swiss company made the scanners used in the Oakland repairs. Its latest generation system, can measure with an accuracy of 6 millimeters from a distance of 50 meters, he says.
With the wreckage mostly mapped, the NTSB will move segments of the bridge by barge to a nearby field. There, a collection of pieces of particular interest will be laid out much like the agency's postcrash airplane reconstructions in hangars.
The next step, say engineering experts, is to make a computer model of the bridge (also known as finite element analysis), dividing it into 52 separate pieces, each critical to the bridge's stability. The more data the better: including original drawings, steel and concrete strengths, temperatures, and the pattern of traffic.
Then, like pulling the crucial block out of a Jenga tower, engineers will remove a piece to see how the bridge would fall. Each collapse scenario will be compared against the maps from the 3-D laser scanner, and the fortuitous video of the event.
Once a match is found, the focus turns to why that particular piece failed. Investigators will be on the lookout for indications that the metal had a slow-growing fatigue fracture, says Abolhassan Astaneh-Asl, an engineering professor at the University of California at Berkeley.
If such a fracture brought down the bridge, controversy over whether the bridge had been properly inspected and maintained may intensify. The head of the state transportation department on Friday angrily denied insinuations that the agency skimped on maintenance following inspections that revealed fatigue stresses.
One longtime bridge collapse expert says he doubts a fracture was the cause. "Brittle fracture problems generally occur in cold weather conditions," says John Hanson, a structural engineering consultant. "A defect of this size that didn't fail when it was 30 below zero would have had to grow [significantly] size since last winter."
Instead, Dr. Hanson says, he would first focus on the construction work that was being done on the deck when the bridge fell. He suspects that the deck, or concrete roadway, played a larger-than-intended role in making the bridge sturdy. Rosenker said in a press conference Sunday that employees of Progressive Contractors Inc. are being interviewed, noting that the company was experienced and had worked on the bridge before.
Mr. Astaneh-Asl also noted that jack-hammering and a heavy asphalt truck on the bridge could have played a role. A respected structural engineer, he shares some of the public frustration with the bridge inspection ratings.
"[Officials] are at such pains to say that 'structurally deficient‚' doesn't mean unsafe," he says. "We should really not have inspections using these terms ... [Just] tell me: Is the deterioration enough to cause someone to become injured or killed?"