BOSTON (AP) – Big Dig officials have repeatedly said taxpayers won’t be saddled with the cost of repairing leaks in the trouble-plagued highway project, but a top federal watchdog isn’t so sure.

Kenneth Mead, inspector general for the U.S. Department of Transportation, said Friday that the Massachusetts Turnpike Authority faces huge hurdles in its effort to force contractors to pick up the tab for the leaks.

“While the authority has said its contractors, and not the taxpayers, will pay to fix all the leaks, we are not entirely confident of this,” Mead told the House Government Reform Committee.

The congressional panel met for four hours at Boston’s federal courthouse to discuss ongoing efforts to patch dripping ceilings, repair defects in the tunnel walls and reassure a jittery driving public that the $14.6 billion system of tunnels and bridges is safe.

Turnpike Chairman Matthew Amorello and officials from project manager Bechtel/Parsons Brinckerhoff said contractors responsible for the leaks will pay for the repairs.

Attorney General Thomas Reilly told the panel that if contractors try to duck their obligation to pay for leak repairs he’ll take them to court.

“Cost recovery is going to go one of two ways. It is either going to be a negotiated settlement or it’s going to litigation,” said Reilly, whose office oversees Big Dig cost recovery efforts.

While the Turnpike typically holds back a portion of payments to contractors until work is finished, it’s not clear that’s enough to cover the cost of the leaks, Mead said. Previous efforts to recover money from contractors have been less than stunning, he said.

But the biggest hurdle will be trying to assign blame for the leaks, according to Mead.

“The primary risk to the taxpayers involves establishing the responsible party,” Mead said. “The problems could be caused by poor construction, design errors, poor oversight, or a combination of those factors.”

Protecting taxpayers is all the more difficult, he said, because some contractors have already been paid in full.

The entire scope of the leak problem in the Interstate 93 tunnels is still unknown. Of 1,750 tunnel wall panels inspected so far, defects – most of them minor – have been found in 109.

Those inspections began after water broke though one faulty wall panel last September, flooding the Interstate 93 northbound tunnel and backing up rush-hour traffic for miles.

Assigning a number to the leaks in the tunnel ceilings has been trickier. As of March, there were 662 leaks identified, but less than half the tunnel’s 9,125 roof sections had been checked and some leaks that had been repaired later re-emerged.

John MacDonald of Bechtel/Parsons Brinckerhoff told the committee the amount of water coming into the tunnels is 20 percent below the norm for a completed tunnel.

Only “a small percentage of roof/wall joints in the I-93 tunnels show signs of low-level leaks,” MacDonald said, adding that Turnpike officials understood sealing those leaks would be a normal part of construction.

“Construction contractors who built the tunnel sections undertook this responsibility as part of finishing their job. They and not taxpayers will pay for the cost of the grouting program,” he told the committee.

After the I-93 tunnels are completed and the leaks plugged, MacDonald said, future leaks will be controlled by the Turnpike Authority as part of normal maintenance.

But Mead said he isn’t convinced contractors will find a way to fix the roof leaks.

“Unless a way is found to permanently seal these leaks, they could remain a long-term problem that leads to higher maintenance costs,” he said.

He also said every wall panel in the tunnels should also undergo a high tech scan to look for hidden defects.

Reilly said the focus must remain on Bechtel/Parsons Brinckerhoff.

“We believe that the responsibility for both the design and the construction rests solely on the shoulders of Bechtel,” he said. “To the extent that there was faulty construction work, they are responsible … not the taxpayer.”

U.S. Rep. Stephen Lynch, D-Mass., a member of the committee, said he wants a legal guarantee from Bechtel/Parsons Brinckerhoff and Big Dig contractors to pick up any maintenance costs over the 75-100-year life of the project that can be blamed on design or construction flaws.

“We’ve got a responsibility to the next generation to give them something that isn’t a constant drain on their resources,” he said. “I wouldn’t want to settle short.”

The tunnels are the centerpiece of the massive highway project, which buried Interstate 93 under downtown Boston and connected the Massachusetts Turnpike to Logan International Airport.