Morris Cohen, 84, an American who spied for the Soviet Union and was instrumental in relaying atomic bomb secrets to the Kremlin in the 1940s, died June 23 in a Moscow hospital.

Russian newspapers reported July 4 that he had died of heart failure and had been buried in a private ceremony so that friends in the Russian intelligence community could attend.

Mr. Cohen, who had called himself Peter Kroger during his spying career, took his most important secret with him: the name of the American scientist who helped pass vital information about the U.S. atomic bomb project at Los Alamos, N.M., in the mid-1940s.

"Thanks to Cohen, designers of the Soviet atomic bomb got piles of technical documentation straight from the secret laboratory in Los Alamos," the newspaper Komsomolskaya Pravda said.

Mr. Cohen, the son of Russian immigrants, was born and raised in New York City. He joined the American Communist Party in 1935 and later went to Spain to fight for the left-wing Abraham Lincoln Brigade in the Spanish Civil War. While recovering from wounds, he was recruited by Soviet intelligence to spy for Moscow in the United States.

In July 1945, just before the first test of the plutonium bomb at Los Alamos, Mr. Cohen, then in the Army, and his wife, Lona, arranged to get detailed information about the bomb from a physicist participating in the top-secret Manhattan Project. The physicist gave blueprints to Lona Cohen in Albuquerque.

The information was passed to Moscow 12 days before the bomb test in the New Mexico desert. Soviet dictator Josef Stalin ordered a crash program and exploded a similar plutonium bomb four years later.

According to news reports in the 1990s, the information the Cohens got from their unidentified source probably was more important than secret information passed on by another atomic scientist working in Los Alamos, Klaus Fuchs, who was arrested in Britain in 1950.

The Cohens moved to New Zealand, changed their names to Peter and Helen Kroger and moved in 1954 to London, where they started a new intelligence network posing as rare book dealers. The network lasted for seven years before British intelligence exposed it.

They were arrested in 1961 and sentenced to 20 years in prison. The couple was exchanged in 1969 for British teacher Gerald Brooke, arrested in Moscow by the KGB for distributing anti-Communist propaganda.

Lona Cohen died in 1992.