US law enforcement and intelligence officials told the US Congress on Tuesday that eavesdropping on electronic communications had stopped terrorist plots against New York City's subway system as well as the New York Stock Exchange (NYSE).

"In recent years these programs, together with other intelligence, have protected the US and our allies from terrorist threats across the globe, to include helping prevent potential terrorist events over 50 times since 9-11," NSA chief General Keith Alexander (pictured above) told the House of Representatives Select Committee on Intelligence.

The NSA has been in the political hot seat, since former agency contractor Edward Snowden leaked the existence of the PRISM surveillance program to the Guardian and Washington Post newspapers. PRISM monitors massive amounts of electronic communications, under secret court order, between the US and foreign countries in an effort to uncover terror plots.

Plots in New York City

FBI Deputy Director Sean Joyce, who also testified on Tuesday, said that the NSA program had uncovered contact between a used-auto parts salesman in Kansas City, Missouri and a "known extremist in Yemen." With that information, the FBI was later able to uncover a bomb plot against the NYSE, according to Joyce.

The FBI deputy director also said that in 2009, the NSA uncovered an email from a Pakistan-based terrorist sent to a suspect in Denver, Colorado. The suspect later traveled to New York City, where he was arrested with a bomb-making component. He later confessed to plans to target the city's subway system.

Google requests disclosures

The Internet giant Google, meanwhile, has filed a request with the Foreign Intelligence Surveillance Court to disclose the number of national security requests it received from the federal government. The court is responsible for issuing the warrants that govern the NSA's surveillance activities, under the Foreign Intelligence Surveillance Act (FISA).

Other Internet companies - such as Facebook and Microsoft - have been allowed to publish the national security requests as part of an aggregate number, which also includes requests related to criminal cases.

"However, greater transparency is needed, so today we have petitioned the Foreign Intelligence Surveillance Court to allow us to publish aggregate numbers of national security requests, including FISA disclosures, separately," Google said in a press release.

"Lumping national security requests together with criminal requests - as some companies have been permitted to do - would be a backward step for our users," Google added.

slk/av (Reuters, dpa)