BOSTON -- A yearlong study by federal investigators into the cause of the Interstate 90 tunnel connector ceiling collapse that killed a Boston woman in July 2006 has led to the conclusion that contractors and builders probably used the wrong kind of epoxy glue to secure ceiling panel anchors.

Bruce Magladry, director of the National Transportation Safety Board Office of Highway Safety, said the 20 anchors, or bolts, that were holding more than 52,000 pounds of concrete ceiling panels in place in the connector tunnel were secured by an epoxy that had short-term strength but which was poor in the long term.

"As an analogy, think of the glue on an adhesive label," Magladry told NTSB board members at a meeting Tuesday, telling them it yields over long-term pressure.

Magladry said the ceiling design "failed to account for creep," the process by which some epoxy adhesives slowly fail over time under heavy loads. He said had an inspection system been in place, officials would have been able to identify safety issues before the fatal accident occurred.

A Boston woman, Milena Del Valle, 39, was killed when she and her husband, Angel, were on their way to Logan International Airport on July 10. Twenty of the concrete ceiling panels -- 12 tons -- fell on their car, crushing her. Angel Del Valle escaped with minor injuries.

"Although the epoxy used in the tunnel had acceptable short-term strength, it was incapable of supporting much lower loads over an extended period of time," Magladry said. "If any of the entities involved in the ceiling design and installation had considered creep as a possibility, a different epoxy or a different anchoring system would have been used."

Investigators said after the Ted Williams Tunnel was built in 1991 and 1992, concerns arose over the expense of the Big Dig tunnel projects and future maintenance costs. They said in 1995, the Federal Highway Administration examined alternate designs and a new approach was used in the I-90 connector tunnel.

Contractors decided to secure the heavy concrete ceiling panels with bolts and secure them with epoxy adhesive, a less costly approach than had been used in the Ted Williams Tunnel, which had more "aesthetically pleasing" porcelain covers over the concrete panels.

Investigators said there weren't any problems with the new design itself, but tests revealed that the epoxy chosen to fasten the bolts was a "fast set" adhesive that was not going to be strong enough to hold the panels up over time, especially because of air bubbles, or voids, that were created between the glue and the bolts during installation. The bubbles reduced the effectiveness of the glue by about 40 percent.

"I think it's safe to say this epoxy is inappropriate in all circumstances," NTSB investigator Mark Bagnard said.

Although the builders conducted tests to determine whether the bolt-glue system would be safe, NTSB investigators said their "calculations were correct but failed to take creep into account."

They added that the use of such adhesives in large tunnels was "atypical" and usually only for light-weight ceiling panels. They said that the tunnel builders and contractors also did not provide for any long-term performance schedules or inspections for the system once they built it.

Once the bolts did begin to give way, federal investigators said, Massachusetts Turnpike Authority officials had ample opportunity to perform tunnel inspections. Had they done so, NTSB engineers said, they likely would have detected the safety issues, especially since two workers noticed the bolts giving way in 1999 and again in 2001. In both cases, NTSB investigators said, tunnel builders thought the loose bolts were "isolated instances" caused by "overtorquing of nuts" when the bolts were screwed into the concrete.

The Massachusetts Turnpike Authority performed no formal inspections of the tunnel ceiling systems from January 2003 to July 2006, engineers said, adding that even a "cursory exam would have shown that the structural integrity (of the tunnel) was compromised."

The Turnpike Authority told investigators it had failed to inspect the tunnels during that time period because it had to build a database first and train staffers on how to perform the inspections. They said they were also waiting for federal highway reviews.

"Had the MTA inspected the super-ceiling ... the anchor creep would likely have been detected," NTSB engineer Dan Walsh said.

The I-90 tunnel accident prompted the closure of highways and tunnels across the state while investigations and repairs took place. More than $54 million has been spent on repairs and the state attorney general is expected to decide whether criminal charges will be filed in the case. Del Valle's family is also planning to file a wrongful death suit against the Massaschusetts Turnpike Authority and other companies associatied with the tunnel construction.

Dozens of building, contracting and design companies were involved in the $14 billion Central Artery Project, also called the Big Dig, which took decades to complete. It is considered the costliest and most complicated U.S. public highway project in the nation's history.

The Federal Highway Administration, the Massachusetts Highway Department and the Massachusetts Turpike Authority oversaw construction and the primary contractor was Bechtel/Parsons Brinckerhoff.

The NTSB issued a set of recommendations as a result of the tunnel accident investigation, including calling for federal safety inspections of all U.S. tunnels. Right now, U.S. highway officials do not have the regulatory authority to inspect tunnels that are not federally funded.

When asked if the NTSB investigators now believe the Big Dig tunnels are safe following extensive repairs, one declined to respond, saying he would defer the question to Massachusetts Turnpike Authority officials.