Prism did not search for keywords -

The US government's PRISM Internet programme does not search for keywords, but snuffles around looking for dodgy email addresses and phone numbers.

Top lawyers of the country's top spooks including the National Security Agency (NSA) and Federal Bureau of Investigation told a public hearing that the system does not quite work the way many thought.

They told the hearing hosted by the Privacy and Civil Liberties Oversight Board (PCLOB) that the NSA did not aim to scoop up all web transmissions, but that the surveillance was tailored to track or uncover terror suspects and other threats.

On the matter of NSA email spying, the lawyers said that the NSA figures out what it wants and we get that specifically.

Robert Litt, general counsel at the Office of the Director of National Intelligence, told the hearing that it was targeted collection rather than bulk collection.

However it is important to remember that the spooks are talking about PRISM, which focuses on foreign suspects outside the United States. This system is subject to less virulent criticism than the other main clandestine operation disclosed by Snowden last year which was the NSA's huge telephone metadata program that gathers information on phone calls by most Americans.

The board is conducting a review of PRISM and other NSA programmes, which can draw "upstream" information directly off the internet's backbone.

The NSA argued that such programmes come under a "compulsory legal" umbrella and that untargeted data is not kept.

The default retention period for PRISM data collected from Internet firms is five years. Upstream data is stored for two years, the board was told.

James Baker, general counsel for the FBI told the board that the data collection from Internet companies like Apple, Google and Microsoft is done with the companies' knowledge.