China has been increasingly successful in recruiting former U.S. intelligence officers to pass along government secrets, a trend that is getting more attention as the Trump administration attempts to limit Beijing’s spy abilities.

At least four Americans have pleaded guilty or were convicted of espionage or attempted spying on behalf of the Chinese over the past year. Kevin Mallory, who was convicted of espionage charges and lying to the FBI, was sentenced to 20 years in prison Friday.