US surveillance foiled 50 plots, says spy chief Alexander
The head of America's electronic spying agency has told Congress surveillance programmes leaked by Edward Snowden helped thwart 50 attacks since 2001.
Army General Keith Alexander, director of the National Security Agency (NSA), told the House intelligence committee the snooping operations were critical.
Plans to attack the New York Stock Exchange were among those foiled, the panel heard.
Meanwhile, Mr Snowden's father urged his son not to commit "treason".
It was Gen Alexander's second public appearance before Congress since the internet and telephone surveillance programmes were exposed this month.
Snowden to face 'justice'
"In recent years these programmes, 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," he said.
Ten of those plots had been intended for the US, the panel heard.
Sean Joyce, deputy FBI director, told the committee one of the cases had involved a "nascent" plot to bomb the New York Stock Exchange.
Mr Joyce said a tip from the NSA had led the FBI to arrest Khalid Ouazzani of Kansas City, Missouri, on conspiracy charges.
Ouazzani admitted in a federal court in 2010 plotting to provide material support to a terrorist organisation, but he was not charged with the alleged stock exchange plot.
Mr Joyce also said the surveillance had been essential to disrupting the 2009 conspiracy to bomb the New York City subway, resulting in the capture of that plot's mastermind, Najibullah Zazi.
Asked what was next for Mr Snowden, Mr Joyce said simply: "Justice."
The former NSA contractor fled to Hong Kong shortly before the scandal broke. His leaks revealed that US agencies had systematically gathered vast amounts of phone and web data.
On Tuesday, Google challenged the US government's gagging order on its internet surveillance programme, citing a constitutional right to free speech.
The web firm asked the secretive US Foreign Intelligence Surveillance Court to allow it to publish the number of government requests in the name of national security for data about its customers.
Tech firms including Microsoft, Facebook, Apple and Yahoo have released limited information about the number of surveillance requests they receive.
But they are currently not allowed to show the split in numbers between requests related to national security and to criminal investigations.
Meanwhile, Mr Snowden's father, Lon Snowden, said in an interview with Fox News: "I hope, I pray and ask that you will not release any secrets that could constitute treason."
He urged his son to come home and "face justice".
Edward Snowden has vowed to fight any extradition attempts by the US.