Robert S. Mueller III, the director of the Federal Bureau of Investigation, warned Wednesday that dismantling the National Security Agency’s once-secret program that is keeping records of billions of domestic phone calls by Americans would slow down investigators as they seek to stop terrorist attacks.
Testifying before the Senate Judiciary Committee, Mr. Mueller addressed a proposal to require telephone companies to retain calling logs for five years — the period the N.S.A. is keeping them — for investigators to consult, rather than allowing the government to collect and store them all. He cautioned that it would take time to subpoena the companies for numbers of interest and get the answers back.
“The point being that it will take an awful long time,” Mr. Mueller said.
Gen. Keith B. Alexander, the director of the N.S.A., had hinted at a House hearing on Tuesday that he was evaluating changes to the domestic calling log program and that he would eventually report back to Congress on the advantages and disadvantages of changing it. He also raised the issue of “speed in crisis” as a major detractor.
In his testimony, Mr. Mueller provided more details about why national security officials were reluctant to take such a step. First, he said, under current law companies are not required to retain such records, and some dispose of them much sooner than five years. Second, rather than being able to instantly query the complete database to see who a suspect has been in contact with, he said, investigators would have to present legal paperwork to a half-dozen carriers and wait for them to gather and provide the records.
“In this particular area, where you’re trying to prevent terrorist attacks, what you want is that information as to whether or not that number in Yemen is in contact with somebody in the United States almost instantaneously so you can prevent that attack,” he said. “You cannot wait three months, six months, a year to get that information, be able to collate it and put it together. Those are the concerns I have about an alternative way of handling this.”
Mr. Mueller did not explain why it would take so long for telephone companies to respond to a subpoena for calling data linked to a particular number, especially in a national security investigation.
Lawmakers also pressed Mr. Mueller to explain what attacks, if any, have been prevented by the N.S.A. program, which came to light following recent leaks by Edward J. Snowden, a former N.S.A. contractor.
Mr. Mueller referred — but in greater detail than had been provided at Tuesday’s hearing — to newly declassified information linking the program to a case in which several men in San Diego were discovered to have sent about $8,500 to Al Shabab, a terrorist group in Somalia.
Specifically, he said, the N.S.A. identified a terrorist-linked phone number in East Africa and decided to run the number against the domestic calls database, discovering that the suspect number had been in contact with a telephone number in San Diego. Investigators then used other legal authorities to obtain the name and address of the person who used the line in San Diego, and then obtained an individual warrant to start monitoring that line.
“That is one case where you have 215 standing by itself,” he said, referring to Section 215 of the Patriot Act, the statute that a secret court ruling has said permits the vast collection of calling logs.
The domestic call log program, which vacuums up “metadata” like which numbers were in contact, and the time and duration of the calls, began as one of the Bush administration’s post- Sept. 11 surveillance programs, without court oversight or statutory authorization.
It later was brought under the authority of the Foreign Intelligence Surveillance Court and rooted to a provision of the Patriot Act that allows the F.B.I. to obtain business records “relevant” to a counterterrorism investigation.
Under recently declassified procedures, the N.S.A. may only consult the database about numbers for which there is reasonable suspicion to believe they are linked to terrorism, and such consultations are audited. Fewer than 300 such numbers were the basis of inquiries in 2012, the agency has said.
Senator Patrick J. Leahy, the Vermont Democrat and chairman of the Senate Judiciary Committee, was not satisfied with that example, asking, “Is it possible to say how many where 215 has been critical? Because we’re talking about billions of phone numbers. How many?”
Mr. Mueller said there had been “anywhere from 10 to 12 where 215 was important in some way, shape or form,” although it was not “instrumental” in all of them, as it was in the San Diego case.
But he warned that counterterrorism investigators need such tools to connect the dots, adding that Congress should decide whether the domestic calling log database program should be retained.
“What concerns me is you never know which dot is going to be key,” he said. “What you want is as many dots as you can. If you close down a program like this, you are removing dots from the playing field,” he said. “Now, you know, it may make that decision that it’s not worth it. But let there be no mistake about it. There will be fewer dots out there to connect” in trying to prevent the next terrorist attack.