A government contractor working for the US National Security Agency (NSA) has been charged with stealing highly classified material, the US Department of Justice said Wednesday.

Thomas Martin III, 51, has been charged with theft of government property and unauthorized removal and retention of classified materials by a government employee or contractor.

Martin, who was secretly arrested on August 27, had a top-secret national security clearance, the Justice Department said in a news release.

The FBI is investigating whether he stole and disclosed highly classified computer codes developed to hack into the networks of foreign governments such as Russia, Iran, China and North Korea, according to the New York Times, which quoted unidentified sources.

The allegations are similar to the case involving Edward Snowden, who was also an NSA contractor in 2013 when he stole material that was later published, exposing NSA surveillance programmes. Martin worked for the same consulting firm, Booz Allen Hamilton, that employed Snowden.

The Department of Justice said searches of Martin's home in Glen Burnie, Maryland, and his vehicle turned up documents and information stored on "various devices and removable digital media."

Much of the materials "contained highly classified information," the news release said. Investigators also located property belonging to the US government valued at more than 1,000 dollars, which Martin allegedly stole.

Among the material found were six classified documents obtained from sensitive intelligence and produced by a government agency in 2014. The documents were produced through "sensitive government sources, methods and capabilities, which are critical to a wide variety of national security issues."

The documents have been reviewed and determined to be properly classified as top secret, the Justice Department said. This means that unauthorized disclosure could be expected to cause "exceptionally grave damage" to US national security.

Lawyers for Martin issued a statement saying they had not seen any evidence that Martin intended to betray his country, according to the Times.

If convicted, he faces a maximum sentence of one year in prison for the unauthorized removal and retention of classified materials and 10 years in prison for theft of government property.

An administration official quoted by the Times said that it did not look like an espionage case.

The official said the case had been handled secretively not in order to keep Martin from becoming "another NSA martyr,” but because it was a continuing law enforcement case and the hope was that Martin would cooperate.

It was unclear when and how authorities first suspected Martin, when they believe he began taking information and whether he passed it to people outside the government.

Snowden faces espionage charges and the possibility of decades in prison if he ever returns to the United States. The NSA secrets he helped disclose revealed details of a massive NSA surveillance programme. He fled the United States before the revelations became public, ultimately landing in Russia, which granted him asylum in 2014.

The White House noted security procedures had been strengthened since Snowden's revelations, including by conducting stronger background checks of employees and reducing the number of people with access to classified information.

"The federal government is reminded of how important it is to be vigilant about protecting the national security of the country and information that is relevant to our national security," spokesman Josh Earnest said, stressing the need for those with security clearances to protect sensitive information.

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