A top U.S. official acknowledged today that the National Security Agency "obviously failed" to protect some of the nation's most important secrets, vowing that within months anyone found responsible for allowing former government contractor Edward Snowden to steal hundreds of thousands of classified documents will be held accountable.

"How soon will we know who screwed up?" the chairman of the Senate Judiciary Committee, Patrick Leahy, D-Vt., bluntly asked a top NSA official during a committee hearing this morning.

"I think that we'll know over weeks and months precisely what happened and who should then be held accountable, and we will hold them accountable," NSA Deputy Director John Inglis said.

Leahy followed up: "Are you taking any steps now to make sure such a screw-up doesn't happen again?"

Inglis said the NSA is taking such steps, insisting, "We have instituted a range of mechanisms - not simply one - to ensure that we would understand and immediately be able to catch someone who tried to repeat precisely what Mr. Snowden did."

But, Inglis added, "We also have to be creative and thoughtful enough to understand that there are many other ways that someone might try to beat the system."

The somewhat tense back-and-forth came only hours after the U.S. intelligence committee declassified three previously-secret documents outlining the NSA's controversial surveillance programs, first detailed through Snowden's disclosures to the press.

Among the documents released is a Justice Department "report" - or white paper - on an NSA program that collects phone numbers, email addresses and other "metadata" related to communications within the United States. The report was sent to certain top lawmakers in December 2009, saying it described "some of the most sensitive foreign intelligence collection programs conducted by the United States government."

The report insisted that the bulk collection program did not collect any content of conversations, and that "the vast majority" of information collected "is never reviewed by anyone in the government."

"We don't even capture … any conversations, so there's no ability - no possibility - of listening to conversations through what we get in this program," a top Justice Department official, Deputy Attorney General James Cole, told the Senate Judiciary Committee.

In the 2009 report released today, U.S. officials said, "The more metadata NSA has access to, the more likely it is that NSA can identify or discover the network of contacts linked to targeted numbers or addresses."

Publicly disclosing any information about the program would be expected to cause "exceptionally grave damage" to national security, said the report.

"We are having a public debate now, but that public debate is not without cost," said Robert Litt, general counsel for Office of Director of National Intelligence. "The information that has been leaked is going to do damage to our ability to protect the nation. we are going to lose capabilities, people are paying attention to this."

If a data breach like the one committed by Snowden would have happened in the private sector, "somebody would have been held accountable by now," said Leahy, suggesting that someone should be have already been fired or at least "admonished."

"If a 29 year old school dropout can come in and take out massive - massive! - amounts of data, it's obvious there weren't adequate controls," Leahy added.

Litt said the NSA's internal inquiry into the matter may find failures "in the individual exercise of responsibility or the design of the system in the first place."

As for the future of the NSA program, lawmakers on both sides of the aisle expressed support for it Wednesday - with increased oversight and increased transparency.

"More about this program probably could be told to the public, and the more that could be told maybe more understanding and less questioning on the part of the public," said the committee's top Republican, Sen. Charles Grassley, R-Iowa.

Looking to tout the program's successes, the deputy director of the FBI, Sean Joyce, said it "played a role" in uncovering the thwarted plot against the New York subway system in 2009. But after Leahy asked whether the program played a "critical role" in identifying the plot led by Colorado bus driver Najibullah Zazi, Joyce acknowledged the FBI was aware of an individual in contact with Zazi before utilizing information from the NSA program.

Still, Joyce said, the NSA program allowed them to identify a terrorist-tied phone number they had not previously known.

"We must have the dots to connect the dots," Joyce said.