The convicted assassin who was the linchpin of the biggest prisoner swap in decades is a member of the most powerful security agency in Russia, the Kremlin acknowledged on Friday, and had served in a special unit with some agents who now guard President Vladimir V. Putin.

The ties help explain Mr. Putin’s determination to free the assassin, Vadim Krasikov, from the German prison where he was serving time for murder. The effort culminated on Thursday when Mr. Krasikov and seven other former prisoners returned to Moscow after an exchange with Western nations that involved 24 adults and seven countries.

This was the first time that Moscow had admitted that Mr. Krasikov had been working for the Russian state in the Federal Security Service, or F.S.B., an agency that is a successor to the Soviet K.G.B., in which Mr. Putin served in the early stage of his career. The F.S.B. was also the agency that was at the center of the negotiations with the C.I.A. about the swap, Dmitri S. Peskov, the Kremlin spokesman, said.

Mr. Putin has not hid his admiration for Mr. Krasikov, who had been jailed in Germany since 2019 for the murder of a Chechen former separatist fighter in Berlin. In an interview in February, Mr. Putin referred to Mr. Krasikov as “a patriot” who was doing his duty by eliminating an enemy of the Russian state.

When the freed prisoners arrived at Vnukovo International Airport in Moscow at about 10:30 Thursday night, Mr. Putin hugged Mr. Krasikov, the first of the freed to disembark the plane.