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US government lawyers have said they will prosecute a former National Security Agency contractor, accused of stealing masses of government data.

Harold Thomas Martin is alleged to have spent more than 20 years collecting data from multiple government agencies, federal prosecutors said.

Court documents say 50 terabytes of data had been seized but it is not clear how much of this was classified.

Mr Martin's lawyer said there is no evidence he betrayed the United States.

Six of the documents found in the 51-year-old's possession were classified as top secret, "meaning that unauthorised disclosure could be expected to cause exceptionally grave damage to the national security of the US", said the Justice Department at the time of his arrest.

Mr Martin was arrested in Maryland in August but officials did not give a motive for the alleged crime.

The FBI said Mr Martin at first denied taking the data, but later admitted removing documents and digital files.

Mr Martin was employed with Booz Allen Hamilton, the same consulting firm that employed Edward Snowden, who gave documents to journalists exposing NSA surveillance practices.

The company said it had immediately fired Mr Martin when it learned of the FBI arrest.

Mr Martin will be charged under the Espionage Act and, if found guilty, faces a hefty prison sentence.

If the case succeeds, it raises serious questions about NSA security, says Alan Woodward, a computer security expert from Surrey University.

"The only extraordinary thing about this story is the volume of data stolen," he said.

"If someone was taking the data out of the NSA over a very long period of time, regardless of motive, it does raise a few questions about how they were able to do that: if someone is removing data habitually you'd expect that to be spotted."