It took years for counterintelligence officials at the C.I.A. and the F.B.I. to put the pieces together and finally solve the puzzle of 1985. Eventually, they realized that they had been confused because there were so many investigative threads to try to follow at once. For example, in August 1985 — just two months after Mr. Stombaugh’s arrest — a K.G.B. officer, Vitaly Yurchenko, defected to the United States, and identified a C.I.A. officer, Edward Lee Howard, as a Russian spy. Mr. Howard escaped to Russia, and Mr. Yurchenko then redefected to Moscow.

American officials were left to wonder whether Mr. Howard had been responsible for the spy losses — and if Mr. Yurchenko’s defection had been genuine or whether he had been ordered by the K.G.B. to defect in order to point them toward Mr. Howard and confuse the Americans.

Ultimately, the C.I.A. and the F.B.I. discovered that it had been difficult to determine what was causing all of the losses because the Russians had more than one mole hidden in the United States government. Both Aldrich Ames, a C.I.A. case officer, and Robert Hanssen, an F.B.I. agent, began spying for the Soviets in 1985. Both gave Moscow the names of Soviets working for the United States, but Mr. Ames and Mr. Hanssen didn’t give the Russians the same exact information.

It was only after Mr. Ames was arrested in 1994 that counterintelligence officials realized there had to be another mole, because Mr. Ames had not known certain things that had been compromised, including the existence of an espionage investigation of a State Department official, Felix Bloch. After Mr. Ames was arrested, the United States secretly began a new mole hunt, which ultimately led to the arrest of Mr. Hanssen in 2001.

Thus, the counterintelligence investigations into the 1985 losses lasted 16 years.

Like their Cold War predecessors, American counterintelligence investigators today face the daunting task of trying to unravel myriad business and personal connections between the Russian government and other Russian entities and people associated with the Trump campaign. It is certain to take officials time to determine which connections are the most significant to their investigation.