From the start, agents knew they were just peering at tea leaves. The two investigators from the FBI's supersecret "Division Five," as the national-security arm is called, reported to the White House on June 3, 1996. In their briefcases they carried classified information that even they didn't fully understand. A surveillance operation launched earlier that year by the satellite spymasters at the National Security Agency had alerted the FBI that the Chinese government might be planning an effort to funnel money into American politics.
The sketchy plan had been pieced together by analysts from various electronic communications in which the Chinese...