NSA Deputy Director John Inglis made the acknowledgment to Senate Judiciary Committee Chairman Patrick Leahy, D-Vt., who accused Obama administration officials of overstating the successes of the far-reaching counter-terrorism program that collects millions of Americans' phone records.
Leahy questioned earlier testimony by NSA Director Gen. Keith Alexander and other senior intelligence officials asserting the phone surveillance helped thwart 54 terrorist events.
Leahy told Inglis he realized after reviewing NSA material that assertion couldn't be made -- "not by any stretch."
He said the NSA material didn't indicate "dozens or even several terrorist plots" had been thwarted by the domestic program.
Inglis admitted the phone-interception program "made a contribution" to discovering 13 domestic plots but was only critical in thwarting one -- a plot he described as a "but for" case.
That case was a September 2009 suicide-bomb plot to wreak havoc in the New York City subway system, Inglis told the committee.
Inglis said the NSA heard Afghan-American suspect Najibullah Zazi talking on the phone about the plot. That interception led the agency to provide authorities with a phone number of a co-conspirator investigators hadn't been aware of.
Zazi and two other defendants pleaded guilty to the plot in 2010.
Inglis acknowledged to Leahy the 54 figure actually involved both the phone-records program and the separate, formerly clandestine PRISM mass electronic-surveillance data-mining program, which allows surveillance targeted at non-citizens abroad.
Leahy upbraided administration officials for conflating the two programs to exaggerate the programs' accomplishments, saying "it needs to stop."
"This bulk-collection program has massive privacy implications," Leahy said. "The phone records of all of us in this room -- all of us in this room -- reside in an NSA database.
"I've said repeatedly, just because we have the ability to collect huge amounts of data does not mean that we should be doing so," he said.
"If this program is not effective, it has to end," Leahy said. "So far, I'm not convinced by what I've seen."
Administration officials defended the collection effort and the PRISM program as critical to U.S. security yet carefully monitored.
"With these programs and other intelligence activities, we are constantly seeking to achieve the right balance between the protection of national security and the protection of privacy and civil liberties," Deputy Attorney General James Cole said. "We believe these two programs have achieved the right balance."
But Cole and Robert Litt, the top lawyer in the Office of the Director of National Intelligence, said the administration was open to re-evaluating the program to regain public trust.
Pending legislation in the Senate would narrow the program's scope.