The paper also noted that an F.B.I. informant learned that a communist acquaintance of the Seborers eventually visited them. The family lived in Moscow and had assumed the surname Smith. The visitor reported back that Oscar and Stuart had said they would be executed for “what they did” if the brothers ever returned to the United States.

In a recent interview, Mr. Klehr said he originally thought the spies were exaggerating their importance. But, he added, the new information on Oscar’s technical knowledge suggested otherwise, although Stuart’s exact role in the spy case remains unclear. “Maybe it wasn’t hyperbole,” Mr. Klehr said.

Last fall, the historians described the Seborers as a Jewish family from Poland that, in New York, became “part of a network of people connected to Soviet intelligence.” Both Oscar and Stuart attended City College, “a hotbed of communist activism,” the historians wrote.

Stuart took a math class there in 1934 with Julius Rosenberg, they reported. In a notorious Cold War spy case, Mr. Rosenberg and his wife, Ethel, were convicted of giving the Soviets atomic secrets. In 1953 they were executed at Sing Sing Prison in Ossining, N.Y., orphaning their two sons, ages 6 and 10.

The scholarly paper, written with John Earl Haynes, a former historian at the Library of Congress, appeared in the September issue of Studies in Intelligence. The journal, a C.I.A. quarterly, is published for the nation’s intelligence agencies as well as academic and independent scholars.

A Times article about the historians’ paper ran on Nov. 23, a Saturday. Four days later, a reporter sent the scholarly paper to Los Alamos and asked if the lab’s archive had any photos of Mr. Seborer or relevant documents.

Two weeks later, on Dec. 10, the lab emailed 10 pages of newly declassified documents from 1956. The material consists mainly of a correspondence between a top security official at Los Alamos and the lab’s branch of the Atomic Energy Commission, a federal agency that oversaw the weapons development site. The letters discussed an F.B.I. investigation of Mr. Seborer’s espionage but gave no specifics on what he may have delivered to Moscow. Instead, the exchange dwelled on the secrets available to him.