The FBI director called out China on its massive espionage effort

China has been increasingly active in foreign espionage, so much so that FBI director Chris Wray said "there's no country that's even close" when it came to compromising vital assets.

Tackling the issue has proven to be a challenge, and the US has significantly underestimated the threat of China's activities, Wray said during an interview with NBC News.

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When the FBI investigates economic espionage, "time and time again, they keep leading back to China," Wray said.

China has long been accused of taking steps to target intellectual property and trade secrets from small startups to major companies.

"The reality is that the Chinese have turned more and more to more creative avenues using non-traditional collectors," Wray said during a Senate hearing in February 2018.

In 2017, the Commission on the Theft of American Intellectual Property published a report saying China violated intellectual property rights more than any other country, and that it was at least partially responsible for a $600 billion hit to the US economy.

One method China employed in the past was to acquire US-based companies. In 2016, one of the lead suppliers of military aircraft for China, Aviation Industry Corporation of China (AVIC), scooped up a small and unprofitable aerospace company based in California.

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"What China is doing with AVIC is making sure they have access to technologies that they wouldn't have otherwise," Tang Energy CEO Patrick Jenevein said in Forbes. That practice is fairly common in business, but China's involvement earns additional scrutiny.

China's activities do not appear to be limited to economic espionage. China has somehow acquired defense industry designs, such as a type of thermonuclear warhead engineered for submarine missiles.