A Mexican researcher has pleaded guilty to acting on behalf of a foreign official to gather information about an unidentified man living in Florida, authorities said Wednesday - a case that alarmed national security officials because the target of the researcher’s surveillance effort had provided U.S. officials with information about Russian intelligence work.

The Justice Department said Hector Alejandro Cabrera Fuentes, 36, acted at the direction of a person he believed was working for the Russian government, taking orders to have an associate lease a condo in a building in the Miami area where he could gather information about the informant.

Much remains murky about the case, but the guilty plea marks another instance in which the FBI has captured someone in the United States apparently working at the bidding of Russian intelligence services.

Fuentes had worked as a researcher at the National Heart Center in Singapore but traveled repeatedly to Moscow, where he had a wife and two children who were having difficulty getting permission from the government to travel outside the country, according to U.S. officials. They said Fuentes also appears to have a second marriage to another woman, who was allegedly involved in the surveillance of the Florida man.

Prosecutors said a Russian official pushed Fuentes to gather information about the informant, who was then living in Miami. Fuentes’s communication with the Russian official was “consistent with the tactics of the Russian intelligence services for spotting, assessing, recruiting, and handling intelligence assets and sources,” the Justice Department said.

In February 2020 Fuentes traveled to Miami and visited the condominium complex where the informant lived. During that visit, Fuentes’s traveling companion - his other wife - took a close-up photo of the informant’s car, according to court papers.

The surveillance effort unraveled when a security guard at the complex stopped Fuentes, questioned him, and told the couple to leave the property.

Two days later, they were detained at Miami International Airport, where they had planned to board a flight to Mexico City. U.S. Customs and Border Protection inspected the wife’s phone and discovered “a close-up image of the license plate” of the informant’s car in a folder of recently deleted photographs.

Fuentes later told FBI agents that he had repeated contact with a person he believed worked for the Russian government, who had contacted him in May when he was visiting his Russian family. Over a series of discussions, the official made clear he knew about the difficulty Fuentes’ wife was having trying to leave Russia, and said “we can help each other,” according to court papers in the case.

He pleaded guilty Tuesday to violating a law that requires individuals to notify the Justice Department that they are working as an agent for a foreign government, in this case Russia, authorities said.

Fuentes faces a statutory maximum of 10 years in prison at his sentencing, which is scheduled for May.