The National Security Agency’s power to scrutinize Americans’ telephone records has helped thwart more than 50 potential terrorist plots, including plans to bomb both the New York City subway system and the New York Stock Exchange, officials said Tuesday as they defended the controversial snooping program to a panel of lawmakers.

In a rare public hearing, top officers from the NSA, Justice Department and the Department of National Intelligence provided a detailed description of the secretive electronic surveillance program created in the wake of the Sept. 11, 2001, terrorist attacks. The program was leaked earlier this month by disgruntled government contractor Edward Snowden.