Americans' email and online chats can be monitored without authorization by any National Security Agency analyst using a computer program known as "XKeyscore," according to NSA leaker Edward Snowden, who shared the information with the Guardian in a story published Wednesday.
XKeyscore is described by the NSA in training materials as its "widest reaching" means of gathering information from the Internet, according to the Guardian. Using the program, "analysts can also search by name, telephone number, IP address, keywords, the language in which the Internet activity was conducted or the type of browser used."
NSA analysts "can also use XKeyscore and other NSA systems to obtain ongoing 'real-time' interception of an individual's Internet activity," the Guardian said.
The NSA documents contend that by the year 2008, 300 terrorists had been captured using information gathered with XKeyscore, the newspaper said.
At a press briefing Wednesday, White House spokesman Jay Carney said that "some of the claims made in that article are false."
House Intelligence Committee Chairman Mike Rogers, R-Mich., and ranking member Dutch Ruppersberger, D-Md., released a joint statement saying "The latest in the parade of classified leaks published today is without context and provides a completely inaccurate picture of the program."
XKeyscore, they said, "does not target American citizens. Further, the program referenced in the story is not used for indiscriminate monitoring of the Internet, as many falsely believe. Rather, the program is simply a tool used by our intelligence analysts to better understand foreign intelligence, including terrorist targets overseas."
Snowden referenced XKeyscore, although not by name, in June, when his first interviews with the British newspaper were published. In a Guardian video interview, Snowden said, "I, sitting at my desk, certainly had the authorities to wiretap anyone, from you or your accountant, to a federal judge, to even the president if I had a personal email."
Nor is this the first public mention of XKeyscore. Earlier this month, German intelligence agencies said the NSA gave them XKeyscore to use, according to documents seen by Der Spiegel reporters, with the program meant to "expand their ability to support NSA as we jointly prosecute CT [counter-terrorism] targets."
XKeyscore was also described earlier this year in "Deep State: Inside the Government Secrecy Industry," a book by Marc Ambinder and D.B. Grady published earlier this year.
"At Fort Meade, a program called XKeyscore processes all signals before they are shunted off to various 'production lines' that deal with specific issues," the authors wrote.
David Brown, who wrote "Deep State" using D.B. Grady as a pen name, told NBC News that what is "surprising everybody" with the XKeyscore information is "just how easily low-level analysts can access the data. The impediments that are supposed to be there really aren't."
"I like to think of it as plumbing," he said. "The pipes come in through XKeyscore, which then diverts the data through different channels, because there's just an awful lot of data."
Another NSA tool, called DNI Presenter, the Guardian said, lets an analyst who uses XKeyscore "read the content of Facebook chats or private messages," as well as the content of stored emails. Facebook declined to comment to NBC News about the report.
The amount of information gathered using XKeyscore is "staggeringly large," the newspaper said. "One NSA report from 2007 estimated that there were 850 billion 'call events' collected and stored in the NSA databases, and close to 150 billion Internet records. Each day, the document says, 1-2b billion records were added."
The Guardian said the XKeyscore training materials shared by Snowden show how easy it is for analysts to use it "and other systems to mine enormous agency databases by filling in a simple on-screen form giving only a broad justification for the search. The request is not reviewed by a court or any NSA personnel before it is processed."
If that claim is true, it may conflict with U.S. legal requirements for performing digital surveillance of Americans, that the NSA obtain a warrant first from the secret Foreign Intelligence Surveillance Act (FISA) court.
"As we've explained, and the intelligence community has explained, allegations of widespread, unchecked analyst access to NSA collection data are false," said White House spokesman Carney.
"Access to all of NSA's analytic tools is limited to only those personnel who require access for their assigned tasks," he continued. "And there are multiple technical, manual and supervisory checks and balances within the system to prevent those who don't have access from achieving that access."
The Senate Judiciary Committee Wednesday held a hearing on the oversight of FISA surveillance programs, which may have something to do with the timing of the Guardian's story. Also underway Wednesday was the annual Black Hat hackers' conference in Las Vegas, where Gen. Keith Alexander, head of the NSA, spoke to a somewhat hostile crowd.
"The assumption is that people are out there just wheeling and dealing, and nothing could be further from the truth," he said in a speech there Wednesday. "We have tremendous oversight in these programs … You know that we can audit the actions of our people 100 percent in this case, and we do that."
Congressmen Rogers and Ruppersberger echoed Alexander's remarks, saying the Guardian story "also once again ignores the legal constraints, comprehensive training, and layers of oversight built into all NSA programs. Every search on the program by an NSA analyst is fully auditable to ensure it is done within the law."
But at the Senate hearing in Washington, D.C., Sen. Patrick Leahy, D-Vt. and chairman of the committee said, "We need straightforward answers. I'm concerned we're not getting them.
"I think the patience of the American people is beginning to wear thin, but what has to be of more concern in a democracy is the trust of the American people is wearing thin."
Also on Wednesday, Director of National Intelligence James Clapper released declassified documents relating to the government’s collection of Americans' telephone data, one of the first disclosures by Snowden in June.
Jameel Jaffer, the American Civil Liberties Union's deputy legal director, said in a statement that the Guardian's "latest revelations make clear that the government's surveillance activities are far more extensive and intrusive than previously understood, and they underscore that the surveillance laws are in desperate need of reform."
Meanwhile, Snowden remains in Russia, where he has spent more than a month in the transit zone of Moscow’s Sheremetyevo airport, as he seeks temporary asylum there.
NBC News' Frank Thorp and correspondent Yannick LeJacq contributed to this story.
This story was updated at 6:25 p.m. ET Wednesday.