Secret US surveillance has foiled more than 50 terror plots since 2001 including a planned bomb attack on the New York Stock Exchange, a US spy chief said, defending leaked programs.
National Security Agency director General Keith Alexander described four thwarted plots, including a plan to bomb the New York subway described as "the first core Al-Qaeda plot since 9/11, directed from Pakistan.''
Alexander, FBI deputy director Sean Joyce and others defended the digital snooping, which they insist has kept America safe since 2001, but which has come under global criticism following leaks of classified details.
"These programs are immensely valuable for protecting our nation and securing the security of our allies,'' Alexander said, in the government's latest bid to deflect criticism of its phone and Internet monitoring.
"In recent years the information gathered from these programs provided the US government with critical leads to help prevent over 50 potential terrorist events in more than 20 countries around the world.''
Alexander said at least 10 of the threats were "homeland-based.''
He told the House Intelligence Committee that most details were classified and would not be made public.
But in an effort to win political support for the programs details of four incidents, including the New York Stock Exchange plot, were released.
Joyce said a tip from the NSA, which had traced international phone calls from terror suspects to Kansas City, led the FBI to get a court order to begin electronic surveillance on one Khalid Ouazzani.
They then determined that he was working with co-conspirators on a "nascent'' plot to bomb the NYSE.
"Ouazzani had been providing information and support to this plot,'' Joyce said. "The FBI disrupted and arrested these individuals.''
In May 2010, Ouazzani pleaded guilty to conspiring to provide material support to Al-Qaeda. But in a statement at the time of the guilty plea, the FBI made no mention of a plot to bomb the stock exchange.
Alexander and other officials appeared at Tuesday's hearing to address a firestorm after rogue defense contractor Edward Snowden pulled back the curtain on a vast electronic surveillance program.
According to material leaked to the Guardian newspaper, the NSA gathers the phone records or millions of Americans and monitors of digital communications like email and online chats from private servers.
While some lawmakers and civil rights groups say US intelligence agencies have trampled on civil liberties by indiscriminately targeting millions of American and foreign nationals, others have rushed to its defense.
"Without them, I fear we will return to the position where we were prior to the attacks of September 11th, 2001, and that would be unacceptable for all of us,'' said committee chairman Mike Rogers.
Alexander agreed. "At the end of the day, we need these tools,'' he said.
"I would much rather be here today debating this point than trying to explain how we failed to prevent another 9/11.''
Officials pointed to the need to gather a mound of data from which to procure information on specific terror suspects.
"If you're looking for a needle in the haystack, you have to have the haystack first,'' Deputy Attorney General James Cole testified.
"While it sits there, it is used sparingly,'' he said.
According to Cole, the database was queried 300 times last year, and only after a secret court created by the Foreign Intelligence Surveillance Act determined there was "reasonable articulable suspicion'' to pursue a suspect.
Senior Democratic lawmaker Dutch Ruppersberger said Snowden's actions had given "the terrorists a really good look at the playbook that we use to protect our country.''
Alexander agreed, saying the leak caused "irreversible and significant damage to this nation.''