The FBI has interviewed visa holders in more than 25 U.S. cities suspected of hiding their Chinese military memberships, the Justice Department said on Thursday, as part of what experts called the biggest known crackdown on the theft of U.S. know-how in more than 40 years of Sino-U.S. relations, Reuters reports.

The Justice Department announcement likely will fuel tensions between the world’s two largest economies that have grown since the Trump administration ordered China to shutter its consulate in Houston, Texas, by Friday.

The administration has intensified charges that China uses cyber operations and espionage to steal U.S. technological, military and other know-how in a strategy to supplant the United States as the world’s leading financial and military power. Beijing denied the allegations.

The FBI recently has interviewed visa holders suspected of having undeclared Chinese military affiliations in more than 25 American cities, the Justice Department said.

“These members of China’s Peoples Liberation Army applied for research visas while hiding their true affiliation with the PLA,” the statement quoted Assistant Attorney General John Demers as saying. “This is another part of the Chinese Communist Party’s plan to take advantage of our open society and exploit academic institutions.”

The Chinese Embassy did not immediately respond to a request for comment.

Last month, FBI Director Christopher Wray said almost half of the nearly 5,000 counter-intelligence investigations the bureau is conducting involve China.

Experts called it the largest known crackdown on the theft of American intellectual property since the two nuclear-armed powers began the process that led to the establishment of diplomat relations in 1979.

“This is by far the biggest response by the U.S. to China’s theft of IP (intellectual property) since the opening to China,” said James Mulvenon, an expert on the Chinese military and cyber operations with SOS International, a contractor that supports U.S. government agencies.

The Justice Department said the FBI recently arrested three Chinese nationals for allegedly concealing memberships in the Peoples Liberation Army when applying for visas to conduct research at U.S. academic institutions.

The FBI is seeking to arrest a fourth research visa fraud suspect who sought refuge in the Chinese consulate in San Francisco after being interviewed by the bureau in June, the department said.

U.S. law enforcement cannot enter a foreign embassy or consulate unless invited, and certain top officials such as ambassadors have diplomatic immunity.

FBI checking Chinese military ties of visa holders in 25 cities