“We can help each other,” the Russian official told Mr. Fuentes, according to federal prosecutors.

Soon, according to federal prosecutors, Mr. Fuentes was involved in a conspiracy to collect information about the U.S. government source that would drag his Mexican wife into the plot and lead to the filing of federal charges against him.

Mr. Fuentes would see the Russian official several times in Moscow to plan the operation, according to federal officials.

He told F.B.I. agents that the Russian official told him “not to rent the apartment in Fuentes’s own name and not to tell his Russian family about their meetings,” according to the affidavit.

Mr. Fuentes told the F.B.I. that he believed the Russian official was a member of the Federal Security Service, which is responsible for Russia’s counterintelligence operations, according to the criminal complaint.

During one of their meetings, the official gave Mr. Fuentes a physical description of a U.S. government source’s vehicle.

The Russian official “told Fuentes to locate the car, obtain the source’s vehicle license plate number and note the physical location of the source’s vehicle,” according to federal prosecutors.

The official told Mr. Fuentes not to take a picture of it.

Mr. Fuentes and the Russian official were supposed to meet again in April or May so Mr. Fuentes could give him the information, federal officials said.