Washington (CNN) -- A top U.S. senator said Thursday the "biggest harm" to come from leaks to the news media about the recently foiled al Qaeda airline plot was that it compromised the mole who turned the bomb over to the United States.
Sen. Joe Lieberman, I-Connecticut, who chairs the Senate Homeland Security Committee, said the premature disclosure of the ongoing mission means the informant won't be able to return to Yemen and continue his infiltration of the secretive and elusive terrorist group.
"The good news is this specific plan -- presumably to use an explosive device on an airliner heading into the U.S. -- was stopped. Even better, the device was seized and is being analyzed by the FBI. But the individual involved obviously can no longer be valued to us and that individual had infiltrated into al Qaeda in the Arabian Peninsula (AQAP). So that's the biggest harm that the leak did," Lieberman said after attending a classified briefing with intelligence officials about the plot.
Lieberman didn't say how or why he thought the agent, who was said to be posing as a suicide bomber, might have been able to return to AQAP.
But he did say he expects AQAP to be much more cautious in the future.
They "have got to be upset and insecure that they allowed into their inner circle and, in fact, asked to carry out a mission, an individual who was associated with an intelligence service," Lieberman said. "They're going to be suspicious of everyone around them now and that has consequences, too."
Lieberman said he was "encouraged" that Director of National Intelligence James Clapper ordered an internal review to determine if details of the mission were leaked to The Associated Press by any U.S. intelligence official and added, "whoever was involved in these leaks should feel like they did something really harmful to our national security and I hope we catch them."
Lieberman is just one of several security-focused lawmakers who are frustrated by the leak and want to find out who was responsible.
On Wednesday, an aide to the chairman of the House Intelligence Committee, Rep. Mike Rogers, R-Michigan, said the committee had begun a "preliminary review" that could lead to a full-scale investigation.
Also, Sen. Dan Coats, R-Indiana, pressed the White House to "initiate an exhaustive investigation into all those responsible for these serious national security violations."
"These irresponsible disclosures of highly sensitive information risk American lives, compromise classified operations, and undermine the hard work and sacrifice made by our intelligence officers and partners overseas," said Coats, a member of the intelligence committee.
Sen. Dianne Feinstein, who chairs the Intelligence Committee, said she thinks the leaks shouldn't have happened. Earlier this week, after a classified briefing, the California Democrat said an investigation is needed.
"There was an operation in progress," Feinstein said. "The leak is regarded as very serious."
Sen. John McCain, R-Arizona, agreed: "You can think of a lot of leaks that are justified, and that they over-classify things, but this one is pretty clear that this harms our ability to defend the nation."
Despite the nearly unanimous belief that the leak was wrong, one senator told CNN that there may not be much that can be done about it.
"It's just very hard" to prevent leaks "if they share that information with a news agency or the media," said Sen. John Cornyn, R-Texas, who sits on the Armed Services and Judiciary committees. "With our vigorous protections of freedom of the press, that's a place most people are not very comfortable going."