WASHINGTON — Although U.S. intelligence officials have indicated since last summer that the National Security Agency was vacuuming up nearly every American telephone record for counter-terrorism investigations, officials acknowledged Friday that the spy agency collects data from less than a third of U.S. calls because it can't keep pace with cellphone usage.
In a speech last month, President Obama called the bulk collection of telephone records the most controversial part of the debate over security and privacy sparked by former NSA contractor Edward Snowden's leaks of classified material. Obama announced plans to impose greater judicial review on the program and to limit how it can be used.
But the NSA operation now seems far less pervasive than it appeared, raising questions about whether it is as essential a terrorist-fighting tool as the NSA and its supporters have argued.
Rather than sweeping in all U.S. call records, officials said, the NSA is gathering toll records from most domestic land line calls, but is incapable of collecting those from most cellphone or Internet calls. The details were first disclosed by the Washington Post.
The officials, who spoke on condition of anonymity because much of the program remains classified, said they did not correct the public record because they did not want to tip off potential adversaries to obvious gaps in the coverage.
"We didn't want to tell the bad guys to go out and get a cellphone," one senior intelligence official said.
The NSA aims to build the technical capacity over the next few years to collect toll records from every domestic land line and cellphone call, assuming Congress extends authority for Section 215 of the USA Patriot Act after it expires in June 2015.
Once the capacity is available, the agency would seek court orders to require telecommunications companies that do not currently deliver their records to the NSA to do so. The records contain phone numbers, times and lengths of each call, but not the content or anyone's name.
Civil liberties activists said the new disclosure did not change their view that the NSA database of billions of domestic call records was unnecessary and could lead to government abuse.
"I don't find this revelation very reassuring," Jameel Jaffer, deputy legal director of the American Civil Liberties Union, said in an email. "To accept their legal reasoning is to accept that they will eventually collect everything, even if they're not doing so already. They're arguing that they have the right to collect it all."
The NSA declined to discuss the gap. "While we are not going to discuss specific intelligence collection methods, we are always evaluating our activities to ensure they are keeping pace with changes in technology," Vanee M. Vines, a spokeswoman, said in a statement.
Two of the most vocal congressional critics of the NSA program, Sen. Ron Wyden (D-Ore.), who sits on the Senate Intelligence Committee, and Sen. Patrick J. Leahy (D-Vt.), who heads the Senate Judiciary Committee, had no comment Friday, according to their aides. Both of those committees were informed of the coverage gap, officials said.
Other senior officials apparently were not told, however, or did not understand the program's reach.
A federal judge in New York who ruled in December that the government's collection of customer records from telecommunications companies was legal, for example, indicated that he believed the NSA operation covered virtually every domestic call.
"The blunt tool only works because it collects everything," District Judge William H. Pauley wrote. He said the government had invoked legal authority to collect "virtually all call detail records."
And in written testimony to the House Judiciary Committee this week, David Medine, chairman of the Privacy and Civil Liberties Oversight Board, which received classified briefings on the NSA systems and issued a lengthy report to Obama last month, said the program involved "ongoing collection of virtually all telephone records of every American."
Medine did not respond to a request for comment Friday.
By contrast, a presidential task force that included former acting CIA Director Michael Morell and former White House counter-terrorism advisor Richard Clarke hinted at the collection gap in a line buried in its 303-page report on NSA surveillance operations.
"The total amount of data collected and retained in the hypothetical version of Section 215 is much greater than the total amount of data collected and retained in the actual version," it said, adding that the NSA collects "only a small percentage of the total telephony metadata held by service providers."
The task force concluded that the phone records program "has contributed to its efforts to prevent possible terrorist attacks" at home and abroad but "was not essential to preventing attacks."
"One of the things that convinced us that the program couldn't be all that useful was that they weren't doing much with it and they weren't spending much money on it," said a source familiar with the inquiry, who declined to be identified when discussing confidential briefings. "It never appeared to us as a program they had valued very highly."
When White House officials told board members that the phone records were useful in ruling out participation by Americans in international terrorism plots, the board members responded, "How could you possibly clear somebody based on a database that didn't include the majority of the phones in the country?" the source said.
The NSA program was begun without court or congressional approval after the Sept. 11, 2001, terrorist attacks. A former senior NSA official said the agency obtained nearly all domestic call records from late 2001 through at least 2006, when the program was brought under the supervision of the Foreign Intelligence Surveillance Court.
But in recent years, the explosive growth of mobile devices outpaced the NSA's capacity to digest the data. And American use of land line phones has plummeted: Less than half of U.S. households have or use a land line, according to the Centers for Disease Control and Prevention.
In his Jan. 17 speech, Obama said the collection of phone records was "designed to map the communications of terrorists" operating in the United States.
Although he said no abuses had been found, Obama said he would order a transition to end the program as it currently exists to ease concerns of potential abuses. The changes already have begun.
As of Thursday, the NSA must get a judge's approval each time it queries the database, not just the approval of a senior NSA official, as in the past, unless an emergency is underway. In addition, NSA analysts can pursue only phone calls that are two steps removed from a terrorist organization instead of three.
But Obama has found it more difficult to move the database out of government hands, as he also called for.
Telephone companies don't want to hold the vast data cache for the government, and the use of government contractors raises privacy and security concerns. Obama has given the intelligence community and the Justice Department a March 28 deadline to come up with a plan.