Hackers backed by the Russian government stole highly classified cyber secrets in 2015 after an NSA contractor placed the information on his home computer, according to The Wall Street Journal.

Secrets stolen in the hack included information on penetrating foreign computer networks and protecting against cyberattacks, according to the Wall Street Journal. Hackers were able to access the data because the contractor was running antivirus software from Moscow-based Kaspersky Lab, the newspaper reported, cutting anonymous sources.

The information stolen in the breach, considered one of the most significant in recent years, could give the Russian government tips on how to make its networks more secure. The theft occurred in 2015 but wasn’t discovered until spring of 2016, the Journal reported.

The revelation comes amid heightened concerns in Washington that hackers working for the Russian government penetrated US computer networks and tapped social media platforms to meddle with the US 2016 election. Government investigators are examining whether the Russian government may have attempted to influence the electorate, and whether President Donald Trump or anyone working for him was knowingly involved. Trump has repeatedly denied involvement.

The Department of Homeland Security and the Office of the Director of National Intelligence on Election Security have concluded that operatives from two Russian spy agencies infiltrated computers of the Democratic National Committee months before the US national election released hundreds of emails from the party staffers.

Facebook said last month it identified about 500 “inauthentic accounts” that bought $100,000 worth of ads that targeted highly politicized social issues such as immigration, guns and LGBT rights. Social media accounts linked to the Russian government also reportedly attempted to intensify racial tensions during the US elections by regularly sharing content intended to fuel the outrage of their audiences, including videos of police violence against African Americans.

News of the hack also emerges nearly a month after the Trump administration ordered federal agencies to remove Kaspersky Lab products from government computers over concerns the cybersecurity software company might be vulnerable to Russian government influence. Kaspersky has rejected the allegations.

Neither the NSA nor Kaspersky responded to requests for comment.

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