BOSTON -- The Big Dig's largest concrete supplier is under investigation, state and federal officials confirmed Thursday.

NewsCenter 5's Mary Saladna reported that the supplier is accused of delivering substandard concrete and falsifying records.

The concrete in question came from Aggregate Industries at least five years ago.

Officials said the substandard concrete probably does not pose a safety risk for drivers who use the Big Dig tunnels. Attorney General Thomas Reilly said any structural problems created by the material would have probably surfaced by now.

"I want to stress that this is an ongoing investigation. There will be further tests -- both visual and sonar to determine whether or not the durability the strength of the concrete has been compromised. Right now, we have no indication that it is," Reilly said.

Reilly said authorities have found falsified documents proving that Aggregate provided substandard concrete for the project and concrete that had initially been rejected.

The accusations come after officials raided three of Aggregate's plant sites in Massachusetts.

"What we found was a scheme, and that's really what it was, a scheme to just reissue new paperwork for the same concrete. We consider this a violation of the contract, and we are also looking at potential violations of criminal law," Reilly said.

Gov. Mitt Romney praised Reilly for his cost recovery efforts.

"The fact that he's now looking beyond just the contractors to those who are the suppliers of the contractors indicates that he is carrying out a very thorough investigation," Romney said.

Lawyers for Aggregate said the company stands by the quality of the 3.8 million cubic yards of concrete it delivered to the Big Dig.