NSA Director Gen. Keith Alexander said Wednesday that his organization is implementing changes to prevent a leak comparable to that committed by Edward Snowden from happening again. (Associated Press)

Senior government officials on Wednesday continued to defend the National Security Agency’s collection of billions of Americans’ phone records as vital to national security, despite skepticism from some senior lawmakers who pressed for an assessment of the program’s utility.

“We can’t go back to a pre-9/11 moment,” the NSA’s director, Gen. Keith Alexander, told the Senate Judiciary Committee, asserting that ending the bulk collection of data on phone calls would risk leaving the intelligence agencies without information that could avert a terrorist attack.

“There is no other way that we know of to connect the dots,” he said. “. . . Taking the program off the table from my perspective is absolutely not the right thing to do.”

The surveillance program has been under growing scrutiny since its existence was revealed in June through documents leaked by former NSA contractor Edward Snowden. The disclosure has prompted a national debate about whether such bulk collection is lawful and appropriate, and whether its usefulness outweighs the intrusion into Americans’ privacy.

That is because even though the phone-records program harvests only data on the calls’ time and duration and the numbers dialed, such metadata is a powerful tool that can disclose people’s associations, which are potentially sensitive.

“Do we really need to collect so much data on Americans?” said Judiciary Committee Chairman Patrick J. Leahy (D-Vt.). “Just simply because you can do something, does it make sense to do it?”

Leahy, who has introduced bipartisan legislation that he says would end the bulk phone records collection, mused aloud about placing roadblocks on every bridge into Washington. “We’d collect hundreds of illegal immigrants. We would collect huge amounts of illegal drugs. . . . Would we do it? No!”

He noted that NSA officials had testified that the phone program, secretly authorized in 2006 by a surveillance court, was “uniquely valuable” in only one terrorism case.

Alexander said the agency has reviewed its value relative to its cost. He added that fewer than 200 numbers, which must show a link to terrorism, have been run against the NSA’s phone database this year.

Sen. Sheldon Whitehouse (D-R.I.) said ending the program would be akin to “unilaterally disarming” the NSA in an area in which other foreign intelligence agencies are active.

Both Leahy and the panel’s ranking Republican, Sen. Charles E. Grassley (Iowa), also raised questions regarding recent revelations of other NSA surveillance activities. They include a report last week in The Washington Post about the NSA collecting billions of records each day of cellphone locations around the world to track individuals’ whereabouts and relationships for foreign intelligence purposes.

Some of the disclosures, Grassley said, “call into serious question whether the law and other safeguards currently in place strike the right balance between protecting our civil liberties and our national security.”

Alexander said the NSA has taken “41 different actions” to prevent another instance of an employee removing documents from the network, as Snowden did, without authorization. He said he would provide them to the panel by next Wednesday.