Now, that door appears to be closing, with the two nations ramping up their strategic rivalry and each regarding academic visitors from the other with greater suspicion — of espionage, commercial theft and political meddling.

The F.B.I. has mounted a counterintelligence operation that aims to bar Chinese academics from the United States if they are suspected of having links to Chinese intelligence agencies. As many as 30 Chinese professors in the social sciences, heads of academic institutes, and experts who help explain government policies have had their visas to the United States canceled in the past year, or put on administrative review, according to Chinese academics and their American counterparts.

It follows the warning of the F.B.I. director, Christopher A. Wray, at a Senate hearing last year that China presented “a whole-of-society threat on their end” that required a “whole-of-society response.”

In a written response to questions, a State Department official said that American law enforcement believed Chinese intelligence services were increasingly using visiting Chinese scholars to target American citizens for intelligence gathering purposes. The department said it would not discuss the details of specific cases.

The F.B.I. said that it would not confirm or deny any investigations into the scholars’ visits.

The Trump administration has sought to crack down on what it sees as intellectual property theft by Chinese scientists working at American research institutions. Last year, it began restricting visas for Chinese graduate students studying in sensitive research fields and warned biomedical researchers at American universities to beware of Chinese spies trying to steal information from their laboratories.