Russian hackers allegedly used popular antivirus software to steal NSA secrets

— -- U.S. authorities believe Russian-backed hackers successfully stole highly-sensitive U.S. government documents by exploiting a popular antivirus software on the home computer of a man working for the National Security Agency, ABC News has been told by several people familiar with the matter.

The massive theft took place in 2015, the sources said, after the man brought home classified documents from his job at the NSA. One source said the man brought the materials home so he could expand his work-related skills, in hopes of being promoted.

On his home computer, the man was using antivirus software from the Moscow-based company Kaspersky Lab, sources said. Russian-backed hackers were able to target his computer and steal his files through the software, sources added.

The still-unidentified man has been cooperating with federal investigators, according to sources.

In 2013, NSA contractor Edward Snowden disclosed many of the agency's most important surveillance programs and, last year, NSA contractor Harold Martin was arrested by the FBI for allegedly amassing a large cache of highly-classified documents at his home.

Kaspersky Lab's CEO, Eugene Kaspersky, dismissed the recently-disclosed case as a "new conspiracy theory." The company said in a statement that it "has not been provided any information or evidence substantiating this alleged incident."

Today's news comes a month after the Department of Homeland Security directed all 430 departments, agencies and offices comprising the U.S. government to rid their systems of any software made by Kaspersky Lab, one of the world's most renowned cybersecurity firms. U.S. officials allege the company has ties to the Russian government, and they have increasingly expressed concern that foreign hackers could try to utilize Kaspersky Lab software to spy on Americans, steal sensitive files or attack critical infrastructure.

"This action is based on the information security risks presented by the use of Kaspersky products on federal information systems," DHS said at the time. "The risk that the Russian government, whether acting on its own or in collaboration with Kaspersky, could capitalize on access provided by Kaspersky products to compromise federal information and information systems directly implicates U.S. national security."

Kaspersky Lab responded in a statement, saying, "Given that Kaspersky Lab doesn't have inappropriate ties with any government, the company is disappointed with the decision by the U.S. Department of Homeland Security (DHS), but also is grateful for the opportunity to provide additional information to the agency in order to confirm that these allegations are completely unfounded. No credible evidence has been presented publicly by anyone or any organization as the accusations are based on false allegations and inaccurate assumptions."

U.S. officials have yet to publicly present any evidence of concerning links between Kaspersky Lab employees and elements of the Russian government.An ABC News investigation earlier this year found that -- largely through outside vendors -- Kaspersky Lab software has been procured by some segments of the Defense Department, Justice Department and other federal agencies.

As ABC News previously reported, the FBI launched a counterintelligence investigation of Kaspersky Lab several years ago. The agency has recently taken new steps to assess Kaspersky Lab's relationship with Russian intelligence services, including interviewing company employees based in the United States.

Kaspersky Lab CEO Eugene Kaspersky has called U.S. government efforts "extreme."

"Kaspersky Lab is facing one of the most serious challenges to its business yet, given that members of the U.S. government wrongly believe the company or I or both are somehow tied to the Russian government," he recently wrote on his blog. "Basically, it seems that because I'm a self-made entrepreneur who, due to my age and nationality, inevitably was educated during the Soviet era in Russia, they mistakenly conclude my company and I must be bosom buddies with the Russian intelligence agencies ... Yes, it is that absurdly ridiculous."

The Wall Street Journal first reported the new case.