The U.S. government released newly declassified documents yesterday naming an American physicist now living in Britain as a top atomic spy for Moscow during World War II. The papers, including intercepts of Soviet spy messages, describe Moscow's successful penetration of major U.S. government institutions including the White House, War Department and State Department during the Roosevelt and Truman administrations.

One of the cables refers to an agent whom the National Security Agency said was probably Alger Hiss, 91, a former State Department official who has long denied allegations that he spied for Moscow in the 1930s and 1940s.

The physicist, Theodore Alvin Hall, 70, worked at the American nuclear weapons center of Los Alamos, N.M., in 1944 and 1945 and has never been prosecuted. A report in The Washington Post on Feb. 25 stated that Hall was a likely candidate to be the key agent known as "Mlad," based on publicly available NSA and FBI documents.

The NSA yesterday released the new batch of intercepted, decoded Soviet spy messages with NSA notes identifying U.S. officials and other individuals as being the agents mentioned by code names in the cables. The two dozen Soviet agents named in the documents include Lauchlin Currie, a personal aide to President Franklin D. Roosevelt, and May Price, the secretary to influential newspaper columnist Walter Lippmann. Many were investigated for espionage in the early 1950s, and were called to testify before the House Un-American Activities Committee, but were never prosecuted in part because the government was not prepared to reveal that it had succeeded in cracking the Soviet codes.

In notes the NSA identified Hall as the agent known to the Soviets as "Mlad," or "youngster," who helped the Soviet Union acquire the atomic bomb. As with many other individuals named in the notes, the NSA did not explain how it deduced that Hall was Mlad. NSA officials said they reached the conclusion on the basis of analysis of the cables and of other available information.

Soviet documents identify Mlad as one of two agents, together with the British atom spy Klaus Fuchs, who tipped off the Kremlin about the first atomic test in July 1945 and provided a rough outline of the bomb. The new documents also identify Hall's Harvard roommate, Saville Savoy Sax, now dead, as the agent code named "Star" who persuaded Hall first to provide information to Moscow about the atomic bomb project.

The new batch of intercepted cables was decrypted under the so-called Venona program. They describe the scope of Soviet intelligence activities in the United States during the period 1943 to 1945, citing more than 100 American agents and informants, including many whose identities have yet to be established. Agents who have been named so far by the NSA include former War Department official William Ullman, former intelligence officials Jay Joseph Julius and Jane Foster, Harold Glasser of the Treasury Department, George Silverman of the Air Force, Nathan Silvermaster of the Board of Economic Warfare, and Harry Dexter White of the Treasury Department.

All these officials were named as Soviet agents by a former Soviet agent, Elizabeth Bentley, to the FBI and during hours of testimony before the House Un-American Activities Committee in the early 1950s. Most asserted their Fifth Amendment right against self-incrimination when questioned by Congress. White died of a heart attack a few days after denying that he was a Soviet agent.

"The Venona documents should put an end to the argument about many of these cases," said John Haynes, a historian at the Library of Congress who has written extensively about Soviet espionage activities in the United States. "This will be painful to many historians. Bentley has been mocked in many books as a blond spy queen. These documents support what she was saying."

The latest Venona release includes a one-page cable from the Soviet intelligence representative in Washington to Moscow, dated March 30, 1945, that refers to a Soviet agent code named "Ales." An NSA annotation on the document states that "Ales" is "probably Alger Hiss," a former senior State Department official who was accused of spying for the Russians by then-Rep. Richard M. Nixon (R-Calif.).

The Ales cable refers to a Soviet agent working in the State Department who accompanied President Roosevelt to the 1945 Yalta conference and subsequently flew on to Moscow. The detail that the agent traveled to Moscow whittles down the number of possible suspects to fewer than a dozen people.

The March 1945 cable says that Ales gave his Soviet controllers reason to believe that while in Moscow he had met Soviet Foreign Minister Andrei Vyshinsky. It says that Ales had been working for Soviet military intelligence "continuously" since 1935, and was the leader of a small group of agents, consisting primarily of his own relatives. It added that Ales and his entire group had been awarded Soviet decorations in recognition of their work.

Allen Weinstein, a historian who wrote a book about the Hiss case, said the newly released document strengthens the case that Hiss was a Soviet agent.

Hiss issued a statement through his son Tony denying that he was Ales. The statement said that none of Hiss's American accusers had ever alleged that he had received Soviet decorations or had led a spy group made up of his own relatives. It also said Hiss was in Moscow only one night after the Yalta meeting, and that the highlight was a visit to the city's famous subway.

The Hiss case last hit the headlines in 1993 when a leading Russian historian, Gen. Dmitri Volkogonov, announced that he had conducted a search of KGB archives and had found no evidence to support espionage accusations against Hiss. Volkogonov later acknowledged that he had not searched the archives of the military intelligence agency, known as the GRU.

American physicist Hall, contacted yesterday at his home in Cambridge, England, repeated his earlier refusal to confirm or deny that he was the key atomic bomb spy known as Mlad. Hall referred a reporter to a statement issued by his lawyer saying that it would be "detrimental to {Hall's} health to be dragged into controversy over allegations regarding events said to have taken place half a century ago." Hall suffers from cancer and Parkinson's disease.

Both the Justice Department and the FBI have refused to comment on their handling of the Hall case, despite repeated inquiries by The Post over the past month, on the grounds that other documents in the case are still classified.

Richard Rhodes, the author of a Pulitzer prize-winning study, "The Making of the Atomic Bomb," said that Hall appeared to be the Kremlin's second most important agent at Los Alamos yet identified. He said only Fuchs, a German physicist who was a member of the British delegation at Los Alamos, was more significant.

"Clearly, what Mlad was able to supply was of immense value," said Rhodes, noting that it was important to Soviet leaders to obtain the same information about the bomb project from several sources to allay doubts about its authenticity.

Although the Justice Department has refused to disclose why it did not prosecute Hall, former intelligence officials familiar with the investigation note that the government was not prepared to publicly disclose the existence of the intercepted Soviet cables up until very recently. The NSA kept details of the Venona program a tightly guarded secret even though the Soviets knew about the decrypting effort through their agent, H.A.R. "Kim" Philby.

Sen. Daniel Patrick Moynihan (D-N.Y.), who has led congressional efforts to declassify decades-old secrets, including the Venona documents, said that some unfortunate episodes of the McCarthy period of anti-Communist inquiries might have been avoided had the government been prepared to disclose all the information in its possession during the 1950s. "We would all have been much better served to get it all out there, and not go through the tortures of charges and countercharges," said Moynihan.

Other intelligence experts said that the government's unwillingness to permit the Venona documents to be used in court had made it impossible to bring charges against many of the people named by Bentley. They cited the case of Joel Barr, who is identified as a Soviet intelligence agent in the NSA documents. An American citizen, Barr returned to the United States several years ago after living in the Soviet Union for more than three decades.

Hall moved to Britain in 1962 in order to take up a post as a biophysicist at Cambridge University, but has been a frequent visitor to the United States, according to former colleagues, who have met him at scientific conferences. The statement released by Hall's British lawyer depicted him as one of the world's leading authorities in the field of biological X-ray microanalysis.

A former Los Alamos physicist, Arnold Kramish, said that Hall was quite open about his left-wing political convictions while working on the Manhattan Project at Los Alamos where the first atomic bomb was built.

"Everyone sort of laughed it off," he recalled. "It was simply something that everyone accepted about Ted that he was into screwball ideas. . . . He was a mystic, interested in Marxism and eastern religions. I never heard anyone say that he was a spy." CAPTION: THEODORE ALVIN HALL