BOSTON (AP) Prosecutors reached a $458 million settlement with contractors over tunnel defects that caused a fatal 2006 accident but a small contractor that isn't involved in the agreement says it unfairly spares the most responsible party from criminal charges.

Under the deal announced Wednesday, Bechtel/Parsons Brinckerhoff, the multibillion-dollar consortium that oversaw design and construction of the Big Dig, the nation's costliest public works project, agreed to pay $407 million. Twenty-four smaller companies are to pay about $51 million collectively.

Powers Fasteners Inc., a Brewster, N.Y.-based company that provided the epoxy blamed for a ceiling collapse that killed a woman in a car, criticized the decision to allow Bechtel/Parsons Brinckerhoff to avoid criminal charges while prosecutors pursue charges against Powers, a smaller, family-owned business.

"The sheer size of this settlement underlines what we think is the undeniable fact that Bechtel bears the real responsibility for this accident," said Max Stern, a lawyer representing Powers Fasteners in the criminal case.

"After all, Bechtel was responsible for the design, it was responsible for the construction and it was responsible for the inspection of the tunnel, and yet, it escapes all criminal charges."

Powers Fasteners, which has been charged with involuntary manslaughter, has denied responsibility for the tunnel collapse. It is the only one out of 15 companies and agencies that has reached a settlement with the dead woman's family, agreeing to pay $6 million.

State officials could seek additional money from Bechtel/Parsons Brinckerhoff over the next 10 years if a future incident causes more than $50 million in damage. Its liability would be capped at $100 million.

The Interstate 90 tunnel ceiling collapse killed Milena Del Valle, 39, of Boston. She was crushed by 26 tons of concrete as she and her husband drove to Logan International Airport in July 2006. The settlement does not have a direct effect on claims filed by Del Valle's family.

Attorney General Martha Coakley said prosecutors believed they had enough evidence to bring criminal charges against both Powers Fasteners and Bechtel/Parsons Brinckerhoff, but decided not to charge the consortium after its officials said they were willing to negotiate a civil settlement.

Coakley said the culpability of Bechtel/Parsons Brinckerhoff was "different in kind than the actions and omissions of Powers." She also said Bechtel/Parsons Brinckerhoff reacted differently than Powers Fasteners when told there was enough evidence to indict them all.

"Bechtel and Parsons immediately, and I would argue in a very sincere and eager effort, said 'What can we do?'" she said.

She said prosecutors believed that getting the consortium to pay a large civil settlement would serve as a deterrent against shoddy work by other construction contractors.

She also noted that the most the state could recover in a fine from Bechtel/Parsons Brinckerhoff if the company had been criminally charged is $1,000 the maximum penalty for a company charged with manslaughter in Massachusetts.

"We believe that today's global agreement is the best possible resolution. I do not say perfect, but the best possible resolution at this time," Coakley said.

John MacDonald, chairman of Bechtel/Parsons Brinckerhoff, said the settlement "is in the best interests of all concerned."

"We have always said that we take responsibility for our work. We understand and acknowledge with this resolution that our performance did not meet our commitment to the public or our own expectations. Above all, we deeply regret the tragic death of Milena Del Valle in the I-90 tunnel," MacDonald said.

The next two largest contractors behind Bechtel/Parsons Brinckerhoff Modern Continental Construction Co. and Gannett Fleming Inc. were not part of the settlement.

The deal also does not bar the consortium from receiving future government contracts.

The $14.79 billion Big Dig, which had an initial price tag of $2.6 billion, was plagued by problems and cost overruns during the two decades it took to design and build it. It replaced an elevated highway that ran through the heart of Boston with a series of tunnels, ramps and bridges.

The National Transportation Safety Board found that the wrong type of epoxy was used to hold up concrete ceiling panels that collapsed and fell on Del Valle's car. The NTSB concluded the collapse could have been avoided if designers and construction crews had considered that the epoxy holding support anchors for the panels could slowly pull away over time.

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