Eugene Spector, a U.S. citizen jailed for a total of 15 years this week after a Moscow court convicted him of espionage, was found guilty of passing biotechnology secrets to the United States, Russia's FSB security service said on Friday.

Russian state news agencies reported on Tuesday that Spector, who was born in Russia and then moved to the U.S., had been handed a 13-year jail sentence for spying. This was added to an existing bribery sentence and converted into a new 15-year jail term in a maximum security penal colony.

State media did not say how Spector had pleaded to the charges in his closed-door trial.

The details of the espionage case against him were not made public at the time either. But the FSB, the main successor of the Soviet-era KGB, said on Friday that Spector had been acting on behalf of the Pentagon.

"The American, acting in the interests of the Pentagon and a commercial organization affiliated with it, collected and transferred to a foreign party various information on biotechnological and biomedical topics, including those constituting state secrets, for the subsequent creation by the U.S. of a system of high-speed genetic screening of the Russian population," the FSB said in a statement.

The FSB, which usually says if a defendant has confessed, did not say how Spector - who had already been serving a 3-1/2-year sentence in Russia for bribery - had pleaded.

Before his 2021 arrest in the earlier case, Spector served as chairman of the board of Medpolymerprom Group, a company specializing in cancer-curing drugs, state media has said.

In his first court case, Spector pleaded guilty to helping bribe an assistant to an ex-Russian deputy prime minister, according to Russian state media.