Federal investigators are looking into whether vibrations from nearby construction could have shaken loose bolts in a Boston highway tunnel, allowing its massive ceiling panels to collapse on a passing motorist.

The state's criminal probe into that deadly collapse is meanwhile focusing on the manager of the $14.6 billion Big Dig project and whether the company responded to concerns about the bolt system voiced as far back as 1999.

Two Massachusetts congressmen who were briefed on the National Transportation Safety Administration's probe said Thursday that vibrations from nearby construction was one of several possible causes now under investigation, along with possible flaws in the materials and equipment used when the tunnels were built.

"They are looking at every single step and every aspect at this point," said Rep. Michael Capuano, D-Mass.

Inspectors sent into the damaged tunnels and other parts of the Big Dig system after the July 10 collapse quickly focused on the epoxy bolt fasteners anchoring some of the ceiling panels in place.

More than 1,300 of those fasteners failed "pull tests" during the past two weeks, forcing the temporarily shutdown of three stretches of highway, including the Ted Williams Tunnel that runs under Boston Harbor to Logan Airport.

Capuano said NTSB officials were checking whether blasting, the movement of heavy construction equipment or the pounding of metal beams could have shaken the bolts loose, though he said they sounded skeptical that vibrations would be the sole cause.

The NTSB is also analyzing the epoxy used and whether it could have been weakened by freezing temperatures, faulty installation or other factors.

One such factor is whether the diamond-tipped bits used to drill holes for the bolts could have created a smooth surface that did not allow the epoxy to bond properly, said Rep. Stephen Lynch, D-Mass. Carbide bits create a rougher surface that enhances the epoxy's adhesive strength, he said.

Massachusetts officials have stressed that the epoxy bolt fasteners in the damaged tunnel weren't widely used elsewhere in the highway system.

The Big Dig, started in 1991, buried the old Central Artery under downtown Boston and linked Interstate 90 to Logan Airport. The project became the brunt of jokes as its costs soared, delays stretched on, and leaks, falling debris and other problems blamed on faulty construction appeared. Six concrete supplier workers were recently charged with fraud for allegedly concealing that some concrete delivered wasn't freshly mixed.

Federal officials have so far issued 12 subpoenas to Big Dig contractors, subcontractors and others since the collapse, Lynch said.

The subpoenas closely mirror ones issued in a state criminal probe that could result in involuntary manslaughter charges.

State Attorney General Tom Reilly said Thursday that he was focusing on the work of project manager Bechtel/Parsons Brinckerhoff and its limited liability contracts with the state, though he emphasized no one involved was "off the hook."

Some of the 40,000 documents gathered so far in the state probe make it clear that project officials were concerned as far back as 1999 about whether the epoxy anchor bolt system could support the 3-ton ceiling panels.

"Now we have to find out, what did they do about it?" he said.

A spokesman for the company did not immediately return a call seeking comment.

Bechtel/Parsons Brinckerhoff has said it spent years working through the appropriate channels of oversight and that its work was evaluated and reviewed by several government authorities during construction.

Gov. Mitt Romney said Thursday that crews tested two bolts Wednesday night in the Ted Williams tunnel. The bolts, which were rated by their manufacturer to withstand 5,600 pounds of pressure, were pulled at 8,400 pounds of pressure and held. If that pattern continues, it suggests individual bolt failures, but not a system failure, he said.

"That would mean a need for ongoing inspections at a very high level and a frequent level, but it would not mean a requirement to completely reconstruct the ceiling structure in the Ted Williams tunnel," Romney said.

The man the Republican governor has focused much of his criticism on, Massachusetts Turnpike Authority Chairman Matthew Amorello, resigned on Thursday. The Turnpike Authority oversaw the Big Dig project.