The House Oversight Committee has kicked off an investigation into reports the federal government's official software may have been hit by hackers more than two years ago – and the feds may have gone on using it, unaware of the cyberattack.

The vulnerability is believed to have stemmed from the National Security Agency's own encryption algorithm that created a "back door" for hackers, one security expert said.

But Americans may never know the full truth of the matter because security officials say the findings may be too sensitive to national security to share.

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The breach could have compromised data across all major federal government agencies, from the Defense Department and the State Department to the Department of Health and Human Services and the Office of Personnel Management.

"There's a lot of very sketchy stuff here," said Matthew Green, a cryptology expert who works at Johns Hopkins University, the Hill reported.

Green's been working on reverse-engineering the encrypted code that's believed to have been hacked. It's suspected the defect in the government's software originated from a "back door" encryption code created by the National Security Agency that was then allegedly repurposed by foreign cybersecurity hackers, the Hill said.

The defect was first recognized in December when security officials discovered a deliberately altered code.

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And it's a matter of great concern. One security official said the repurposing of code was akin to "stealing a master key to get into any government building," CNN reported.

And Sen. Ron Johnson, R-Wisc., who heads up the Senate Homeland Security and Governmental Affairs Committee, said in the Hill: "It's a very serious problem. It affects everybody's IT systems."

Investigators say a foreign government is most likely the cause of the repurposing, due to the level of infrastructure and intelligence needed to successfully develop and maintain the "back door" entrance.

"Very few people outside of nation states have both of those things," Green said, the Hill reported.

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The code's been in use since 2013, meaning federal agencies could have been compromised for yearsg. Among the information that could have been hacked: Passwords, map networks and other knowledge that would allow the attackers to gain entry into other systems.

"Once adversaries get into a network," said Paul Stockton, the assistant secretary of Defense for homeland defense between 2009 and 2013, the Hill reported, "they're often able to move laterally."