The director of the National Security Agency told Congress on Wednesday that "dozens" of terrorism threats had been halted by the agency's huge database of the logs of nearly every domestic phone call made by Americans, while a senator briefed on the program disclosed that the telephone records are destroyed after five years.
The director, Gen. Keith B. Alexander, who heads both the N.S.A. and United States Cyber Command, which runs the military's offensive and defensive use of cyberweapons, told skeptical members of the Senate Appropriations Committee that his agency was doing exactly what Congress authorized after the attacks of Sept. 11, 2001.
General Alexander said he welcomed debate over the legal justification for the program because "what we're doing to protect American citizens here is the right thing." He said the agency "takes great pride in protecting this nation and our civil liberties and privacy" under the oversight of Congress and the courts.
(Read More: NSA Leak Reports Sloppy and Misleading: Andreessen)
"We aren't trying to hide it," he said. "We're trying to protect America. So we need your help in doing that. This isn't something that's just N.S.A. or the administration doing it on its own. This is what our nation expects our government to do for us."
But in his spirited exchanges with committee members, notably Senator Patrick J. Leahy, Democrat of Vermont, General Alexander said he was seeking to declassify many details about the program now that they have been leaked by Edward J. Snowden, a former N.S.A. contractor who came forward to say he was the source of documents about the phone log program and other classified matters.
Senator Dianne Feinstein, the California Democrat who is chairwoman of the Senate Intelligence Committee, was the first to disclose that the records are eventually destroyed. She said that she planned to hold a classified hearing on Thursday on the program. But at the Wednesday hearing, where testimony about the government's planned $13 billion spending on cybersecurity was largely swept aside for a discussion of the surveillance program, Ms. Feinstein also revealed that investigators had used the database for purposes beyond countering terrorism, suggesting it might have also been employed in slowing Iran's nuclear program.