But preliminary examination of the remains of the bridge show that all 11 steel cables buried in the bridge’s concrete were corroded.

State inspectors also found a rust spot the size of a nickel — the only sign of possible trouble — on a similar bridge several hundred yards away. That led them to recommend closing it to traffic through Saturday’s Coca-Cola 600. This week, the racetrack plans to test the strength of that second bridge, which was built in 1996.

Parent company Speedway Motorsports has hired engineers who reconstruct building accidents to study how part of the pedestrian bridge fell. That investigation could take as long as six months to complete, said Don Idol, a bridge engineer with the N.C. Department of Transportation.

“Engineers are narrowing their focus on corroded steel cables, located inside the pre-cast concrete, as the possible cause of the failure,” said Jerry Gappens, vice president of public relations for the speedway. “Engineers are still trying to determine what caused the corrosion in this isolated area of the span.”

Gappens said weight on the bridge was not a factor in its collapse.

At 11:15 p.m. Saturday, after The Winston night race ended, an 80-foot section of the 320-foot-long walkway sagged from its height of 17 feet to only about five feet above U.S. 29 just outside the racetrack. The section then snapped completely and fell onto the highway.

Rescue workers dispatched 101 people to five area hospitals, some in critical condition.

Idol said debris from the bridge remains at the racetrack. A 70-ton crane and backhoe cleared the debris from U.S. 29 Sunday.

Idol and other state engineers found rust on the half-inch cables embedded in the bridge’s concrete that may indicate the bridge could support less than its designed limit of 100 pounds per square foot. They also noted three to five cracks — each spanning about three feet — in parts of the bridge that did not fall.

State investigators said they are still unsure how water could have gotten into the concrete. Idol could not say for sure whether it was caused by faulty construction.

That’s now up to Speedway Motorsports to figure out. Because the company owns the walkway, no government agency is overseeing the investigation and the track will use its own engineers to determine what went wrong.

Speedway officials issued a news release Monday that said they are “narrowing their focus on corroded steel cables, located inside the precast concrete, as the possible cause of the failure.”

Don Goins, chief engineer for the Department of Transportation, was at the accident site Sunday and Monday. He said all 11 cables were corroded.

Built in 1995, the bridge was designed with a 50-year lifespan. The bridge was built by pouring wet concrete around stretched steel cables to make the slabs. After the slabs dried, the tension on the cables was released, which gives the concrete additional strength.

The pre-stressed concrete slabs in the $1 million bridge were installed by Tindall Corp. of Spartanburg, S.C.

The Department of Transportation inspected the bridge when it was built, but officials did not know the last time the agency had looked at it other than to check the clearance below it. The Department of Transportation inspects state-owned pedestrian bridges at least every two years but has no policy for inspecting 10 privately owned pedestrian bridges throughout North Carolina, Idol said.

State Sen. Fletcher Hartsell, a Concord attorney, told the Associated Press that the tragedy could lead to legislation that changes the state’s inspection process.