How he communicated with his controllers is unknown, as is what specifically he gave the Soviets in terms of atomic secrets. However, it is clear that Moscow mastered the atom very quickly compared with all subsequent nuclear powers.

In the United States under a false name, Dr. Koval initially gathered information about new toxins that might find use in chemical arms. Then his G.R.U. controllers took a gamble and had him work under his own name. Dr. Koval was drafted into the Army, and by chance found himself moving toward the bomb project, then in its infancy.

The Army judged him smart and by 1943 sent him for special wartime training at City College in Manhattan. Considered a Harvard for the poor, it was famous for brilliant students, Communists and, after the war, Julius Rosenberg, who was executed for conspiring to steal atomic secrets for the Soviets.

But Dr. Koval steered clear of all debate on socialism and Russia, Dr. Bloom said. “He discussed no politics that I can recall. Never. He never talked about the Soviet Union, never ever, not a word.”

At City College, Dr. Koval and a dozen or so of his Army peers studied electrical engineering.

Dr. Kramish said the Army unit lived in the Hebrew Orphan Asylum, across from City College, adding that Dr. Koval called himself an orphan. Something else about him stood out, Dr. Kramish said — he was a decade older than his peers, making everybody wonder “why he was in this program.”

Meanwhile, the Manhattan Project was suffering severe manpower shortages and asked the Army for technically adept recruits. In 1944, Dr. Koval and Dr. Kramish headed to Oak Ridge, where the main job was to make bomb fuel, considered the hardest part of the atomic endeavor.

Dr. Koval gained wide access to the sprawling complex, Dr. Kramish said, because “he was assigned to health safety” and drove from building to building making sure no stray radiation harmed workers.