When Boston's Big Dig was still on the drawing board, state and federal transportation officials picked an engineering powerhouse and a smaller, well-established firm to build the forbiddingly complex tangle of tunnels, ramps, bridges and highways.

And then, critics say, the officials stepped back and let the two companies do their job with little or no oversight.

In the weeks since 12 tons of ceiling panels from a Big Dig tunnel fell, crushing a woman to death in a car, critics say one of the key flaws in the project was the way it was managed.

Bechtel Civil Inc., part of San Francisco-based Bechtel Corp., teamed up with the design firm of Parsons, Brinckerhoff Quade and Douglas Inc. to work on the project, starting in 1985. The consortium was known as Bechtel/Parsons Brinckerhoff.

"It's been the fox guarding the hen house," said Sen. Steven Baddour, chairman of the Legislature's Joint Transportation Committee. "They are the only entity that has been with the project from the very beginning, and they've skated."

The $14.6 billion Big Dig, the most expensive highway project in U.S. history, has been plagued by delays, cost overruns, leaks, falling debris and allegations of shoddy workmanship and inferior materials.

Bechtel/Parsons Brinckerhoff has largely avoided blame, despite occasional talk among state officials to ban it from other projects.

Bechtel/Parsons Brinckerhoff required that questions for this story be submitted in writing so they could be reviewed by its attorneys. Nearly a week later, the company still had not provided a response.

In one statement issued after the ceiling collapse, it defended both its construction techniques and oversight.

"Although we believe the project's engineering, construction and quality assurance processes were robust, a comprehensive review — including design, construction, operations and maintenance — is essential to restoring public confidence in this historic project," the company said.

On paper, the Bechtel part of the partnership had the know-how to undertake the Big Dig. Founded in 1898, Bechtel had worked on some of world's premier construction projects, including the Hoover Dam and the Chunnel, the undersea rail tunnel connecting England and France.

But as early as 1996, then-state inspector general Robert Cerasoli was sounding alarms about the close relationship between the Massachusetts Highway Department, then overseeing the project for the state, and Bechtel/Parsons Brinckerhoff.

"MassHighway's near total reliance on the joint venture of B/PB to manage the full range of administrative, preliminary design, design management, and construction management services creates significant and unnecessary financial risk for the commonwealth," Cerasoli's office said in the 1996 letter to Peter Zuk, then the Big Dig's project director for the state.

The inspector general's office said Bechtel/Parsons Brinckerhoff would have little incentive to tell the state about problems if they could be traced back to the companies. Cerasoli also worried about Bechtel/Parsons Brinckerhoff essentially monitoring its own work.

"When the project falls behind schedule, B/PB and the contractors are paid more, not less, to try to bring the project back on track and meet the deadline," the letter said.

In response, Zuk argued that breaking with Bechtel/Parsons Brinckerhoff would be too disruptive.

"It is difficult to see how the delays, inefficiencies, and increased costs associated with hiring a new management consultant would benefit the commonwealth," Zuk wrote.

It was a refrain that would become familiar to Big Dig observers as the project experienced monumental delays and its price ballooned. The initial price tag for the project was $2.6 billion, and the Big Dig was supposed to be completed in seven years. Instead, it took nearly 15 years and $14.6 billion.

The job of recovering some of the cost of the project, including some of the more than $2 billion paid to Bechtel/Parsons Brinckerhoff, was handed to state Attorney General Thomas Reilly. Those efforts are on hold while Reilly's office investigates whether the accident should result in criminal charges.

Several state and federal agencies are trying to determine why the ceiling panels failed in the July 10 accident, and are focusing on the bolt-and-epoxy system that was used to keep the three-ton panels in place.

"It would seem to me that a great deal of the culpability for the roof collapse would fall on Bechtel because they were involved with the design and oversight of construction," said the current inspector general in Massachusetts, Gregory W. Sullivan.

In 1997, the state shifted control of the project from the Highway Department to the Massachusetts Turnpike Authority. With the Turnpike and Bechtel/Parsons Brinckerhoff both overseeing the project, Sullivan said, it has been easier for both to avoid responsibility.

"You get a circle," he said. "When you have two people signing off and something goes wrong, each one points to the other."