The National Security Agency's director says the government's phone and Internet surveillance programs prevented numerous terrorist plots, including one aimed at the New York Stock Exchange.
In a rare open congressional hearing, representatives from the FBI and NSA said nearly 50 plots were prevented by the controversial programs.
The Wall Street plot was one of two plans detailed in their testimony.
Details about the surveillance programs that gather phone records and track the use of U.S-based Internet servers were leaked by contractor Edward Snowden, 29.
Security officials and legislators call his actions a national security violation.
Rep. Michele Bachmann of Minnesota: "How damaging is this to the national security of the American people that this trust was violated?"
General Keith Alexander, NSA Director: "I think it was irreversible and significant damage to this nation.''
Bachmann: ''Has this helped America's enemies?''
Alexander: ''I believe it has, and I believe it will hurt us and our allies."
Last week, intelligence officials described two other separate plots they say were prevented by the surveillance program.
One involved the use of backpack bombs in the New York City subway system.