MINNEAPOLIS — Investigators looking into the cause of the Interstate Highway 35W bridge collapse are likely to focus on two primary causes — vibration and fatigue cracking, the former chairman of the National Transportation Safety Board said in an interview early this morning.

Jim Burnett said they should look at whether vibration from construction work and from a train that was passing under the bridge contributed to the collapse.

"Vibration is one of things that cause cracking to propagate," he said. "They will be looking at that."

Burnett, who is in town for a Republican National Committee meeting, was watching the scene at 5 a.m. with other onlookers at the University Avenue SE. northbound entrance.

He also said he was intrigued by a 2005 study that found signs of "fatigue cracking" in the bridge supports, though he noted that a later report apparently concluded that the bridge was in no immediate danger and did not need major repairs.

"I think that decision is going to come under new scrutiny," he said.

The 2005 study also found that the bridge was "structurally deficient."

"A structurally deficient bridge might be one not adequate for the traffic it takes, but not necessarily dangerous," Burnett said. "But a lot of structurally deficient bridges are dangerous."

Burnett said he believed it would be the state's responsibility to check on the bridge in light of those reports. But he conceded that there could be a federal role and an issue of whether fed standards are adequate.

Lucy Kender, a MnDOT spokeswoman, acknowledged Thursday morning that the state does have primary responsibility for inspecting and maintaining bridges.

When asked about Burnett's concern about the fatigue cracking in the bridge, she said she didn't know whether MnDOT took any special action beyond the routine inspections.