WASHINGTON -- U.S. President Barack Obama's national security team acknowledged for the first time Wednesday that, when investigating one suspected terrorist, it can read and store the phone records of millions of Americans.

Since it was revealed recently that the National Security Agency puts the phone records of every American into a database, the Obama administration has assured the nation that such records are rarely searched and, when they are, officials target only suspected international terrorists.

Meanwhile, at a hacker convention in Las Vegas on Wednesday, the head of the NSA said government methods used to collect telephone and email data helped foil 54 terror plots -- a figure that drew open skepticism from lawmakers back in Washington. "Not by any stretch can you get 54 terrorist plots," said the chairman of the Senate Judiciary Committee, Sen. Patrick Leahy, a Democrat.

More than a decade after the terror attacks of 2001, the phone-record surveillance program has stirred deep privacy concerns on Capitol Hill, where Leahy said Wednesday during an oversight hearing: "If this program is not effective, it has to end," adding that, "So far I'm not convinced by what I've seen."

In the House earlier this month, lawmakers said they never intended to allow the NSA to build a database of every phone call in America, and they threatened to curtail the government's surveillance authority. "You've got a problem," Rep. James Sensenbrenner, a Republican, told top intelligence officials weeks ago.

Sensing a looming shift in the privacy-versus-security cultural calculus, the White House responded: It has ordered the director of national intelligence to recommend changes that could be made to the phone-surveillance program, and President Barack Obama invited a bipartisan group of lawmakers to the White House on Thursday to discuss their concerns about the National Security Agency's surveillance programs.

A White House official says the top Democrats and Republicans on the House and Senate intelligence panels will attend. So will Democratic Sens. Mark Udalland Ron Wyden, two lawmakers who raised the alarm about the NSA's sweeping domestic programs. Two others calling for more NSA oversight, Sen. Dick Durbin of Illinois and Sensenbrenner, will also attend.

The administration has emphasized what it describes as oversight of its activities by the secretive Foreign Intelligence Surveillance Court, by congressional committees and by internal auditors. It has said, for example, that under rules approved by the court, only 22 people at NSA were allowed to approve searches of the phone database, and only seven positions at NSA, a total of 11 people, were authorized to disclose any results believed to be associated with persons in the United States.

Testimony before Congress on Wednesday showed how easy it is for Americans with no connection to terrorism to unwittingly have their calling patterns analyzed by the government.

It hinges on what's known as "hop" or "chain" analysis. When the NSA identifies a suspect, it can look not just at his phone records, but also the records of everyone he calls, everyone who calls those people and everyone who calls those people.

If the average person called 40 unique people, three-hop analysis would allow the government to mine the records of 2.5 million Americans when investigating one suspected terrorist.

The NSA has said it conducted 300 searches of its telephone database last year. Left unsaid until Wednesday was that three-hop analysis off those searches could mean scrutinizing the phone records of tens or even hundreds of millions of people.

"So what has been described as a discrete program, to go after people who would cause us harm, when you look at the reach of this program, it envelopes a substantial number of Americans," said Sen. Dick Durbin, the No. 2 Democrat in the Senate.

John Inglis, the NSA's deputy director, conceded the point but said NSA officials "try to be judicious" about conducting hop analysis.

"And so while, theoretically, 40 times 40 times 40 gets you to a large number, that's not typically what takes place," he said. "We have to compare the theory to the practice."

Such reassurances have done little to quell the sharp criticism from both parties over the once-secret program. Last week saw a close vote in the House on a measure that aimed to kill the phone surveillance program.

On Wednesday, the administration acknowledged some limitations to its sweeping surveillance powers are inevitable.

"We are open to re-evaluating this program in ways that can perhaps provide greater confidence and public trust that this is in fact a program that achieves both privacy protections and national security," Robert Litt, counsel to the Office of the Director of National Intelligence, told skeptical members of the Senate Judiciary Committee.

This newest privacy-vs.-security debate was touched off when former government contract systems analyst Edward Snowden leaked classified documents exposing NSA programs that store years of phone records on every American. That revelation prompted the most significant reconsideration yet of the vast surveillance powers Congress granted the president after the Sept. 11, 2001, attacks.

The administration intended to keep the telephone program a secret, and for more than a decade few in Congress showed any interest in limiting the surveillance. Snowden's leaks abruptly changed the calculus on Capitol Hill.

"We have a lot of good information out there that helps the American public understand these programs, but it all came out late," Sen. Sheldon Whitehouse, a Democrat, said in a rebuke of government secrecy. "It all came out in response to a leaker. There was no organized plan for how we rationally declassify this so that the American people can participate in the debate."

The telephone program is authorized under a provision of the USA Patriot Act, which Congress hurriedly passed after Sept. 11. President George W. Bush's administration said then what Obama's administration says now: that in order to connect the dots, it needs to collect lots of dots.