North Korean hackers stole tens of thousands of documents related to the defense industry — including U.S. fighter jet designs — as part of a much broader cyberattack on South Korea, officials in Seoul said on Monday.

Over 40,000 documents were stolen in breaches of two conglomerates that began in 2014 and were discovered in February, officials said, according to multiple sources.

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The stolen documents included wing blueprints for an F-15 jet fighter taken from Korean Air Lines, a contract manufacturer for the South Korean military.

Spokesmen from both Korean Air Lines and the Defense Ministry said that none of the stolen documents were classified, according to Reuters.

But the attack appears to be part of a much larger campaign by North Korea that was disrupted when the theft was discovered, officials said.

“North Korea turns out to have been preparing for a long time to try to launch a countrywide cyberattack,” the Korean National Police Agency said, according to The Wall Street Journal.

The police say the attack originated at an IP address in Pyongyang that was used in a 2013 cyberattack on South Korea that disabled the computer systems of banks and broadcast companies.

Amid rising tensions between the two nations, there have been a series of unconfirmed reports this year that South Korean government agencies have been infected by malware planted by North Korea.

North Korea has steadfastly denied any involvement in cyberattacks on its neighbor to the south, but experts warn that North Korea is developing significant military and clandestine cyber capabilities.

The country is not seen as a top cyber power yet, but U.S. lawmakers warn that its burgeoning capabilities could be turned on the United States.

The U.S. has blamed North Korea for the cyberattack on Sony, which coincided with the release of a film portraying a fictional assassination attempt on North Korean leader Kim Jong Un.

Last January, the White House authorized a slate of economic sanctions against the country.

In February, President Obama signed legislation mandating sanctions on anyone involved in Pyongyang’s growing cyber arsenal and resurgent nuclear program.

“This is the first time we’ve ever had mandatory sanctions on cyberattacks,” Sen. Cory Gardner Cory Scott GardnerJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Gardner on court vacancy: Country needs to mourn Ginsburg 'before the politics begin' MORE (R-Colo.), who backed the legislation, told The Hill in February. “It’s long overdue.”