His story is extraordinary in many ways. Both as a scientist and as a spy he was a prodigy: he was just 19 years old and one of the youngest members of the staff at the Los Alamos bomb laboratory when he passed his most important information to Soviet agents. Yet he seems to have had clear, well-defined political motives for his actions, and he never in later life disowned or regretted what he had done.
Perhaps more remarkably, although he was identified by the US security services as an agent as early as 1950, they were never able to prosecute him for fear of compromising the source of their information. That same source - the so-called Venona decrypts - also pointed the finger of suspicion at Fuchs, who confessed, and at Maclean, who fled, but the young Hall stood his ground and remained a free man.
If there is a mystery about him, it relates to his later life. In the 1960s he moved to Cambridge to pursue ground-breaking work in biology which earned him an international reputation. Did the US authorities, knowing as they did, beyond doubt, of his past role, really allow him to go without extracting some price?
Hall was born in 1925, the youngest child of a Jewish family called Holtzberg who were in the fur business in New York. When he was still very young the Depression struck and the Holtzbergs suddenly exchanged prosperity for hardship, an experience that left its mark. Young Ted, as he was known, was none the less a spoiled child, adored in particular by a mother who had believed before he arrived that she could have no more children.
As he grew up he came under the influence of his brother Ed, who was 11 years older and a scientist. Ed helped develop a mathematical talent in the boy that soon proved outstanding: in school Ted was promoted into a class three years ahead of his age group and still was able to help other children to follow the lessons. This, and some wartime fast- streaming for promising scientists, helps explain how he came to be a Harvard graduate when he was only 18 and was instantly headhunted for the Manhattan Project.
He was not only precocious as a scientist. The Depression and the rise of Fascism in Europe, with its stong anti-Semitic element, encouraged the boy in radical ideas and here again his brother Ed, who brought home left-wing literature, was a strong influence. At Harvard Ted mixed with young men sympathetic to Communism and became a member of the John Reed Society, dedicated to the ideas of the author of Ten Days That Shook The World.
For the details of how and why he came to be a spy we really only have Hall's word, and experience dictates that it should be treated with caution. He told his story in the mid-1990s to two American journalists, Joseph Albright and Marcia Kunstel, who subsequently recounted it in a book entitled Bombshell: the secret story of America's unknown atomic spy conspiracy (1997).
At Los Alamos in mid-1944 Hall found work on the atomic bomb proceeding apace under the direction of Robert Oppenheimer, and he himself was soon drawn into important, cutting-edge research. He became concerned, he said later, about the consequences of an American monopoly of atomic weapons after the war. Say there was another depression, could a Fascist government emerge in the United States? Could such a government use the bomb to dominate the world? And as for the Soviet Union, which unlike Britain was being excluded from the atom project, was it not an ally, and was it not bearing the brunt of the fight against Hitler?
With a single-mindedness and determination that few of his age could muster, Hall decided to let Moscow into the secret himself. During a short holiday in New York in late 1944, and with the help of a left-wing friend, he made contact with Soviet officials and presented them with a first bundle of documents. Over the ensuing months other bundles followed.
Although Moscow was at first sceptical, the information proved to be extremely valuable, notably for what it revealed about the more sophisticated of the two bomb designs being developed, the plutonium weapon eventually dropped on Nagasaki. It was, moreover, timely, for it seems that Hall supplied his first secrets from Los Alamos before Fuchs did.
In 1950 Fuchs was arrested and convicted as a spy in Britain amid enormous international scandal, and he became known as the man who gave Stalin the atom bomb. The story of the Soviet weapon is of course more complex than that, partly because they had their own scientists who had their own ideas and partly because, as we now know, Fuchs was far from being alone in passing on secrets. Indeed, at least two spies in the Manhattan Project have never been identified.
We know this because of Venona. This was coded telegram traffic between Moscow and its missions abroad during the 1940s, which was intercepted by the US and later partially decoded. Much of the traffic related to intelligence and one of the messages in particular pointed to Hall. Dated 12 November 1944, it stated unequivocally that Theodore Hall, aged 19, a furrier's son and Harvard graduate working at Los Alamos, had met a Soviet agent and handed over a report about the Manhattan Project.
By the time the codebreakers had uncovered this gem, however, it was 1950; Hall had long since left Los Alamos, had severed his connections with Soviet agents and was working quietly in academic research in Chicago. Although FBI agents put pressure on him to confess he gave nothing away, and they could find no other evidence against him beyond the Venona documents.
Since Venona was still yielding fresh secrets at that time and promised to be a counter-intelligence gold mine for many years to come, the US security authorities believed they could not afford to let Moscow know they were cracking the code. (In fact Moscow knew anyway, thanks to Kim Philby, among others.) So it was that, in the expectation that they might catch other fish in future, the FBI let Theodore Hall swim free.
There, in a sense, the story of Hall the spy ends, or at least is put on ice for several decades, and the story of Hall the scientist of international repute begins. By 1950 he had already switched disciplines from nuclear physics to the emerging field of biophysics, where over the coming 20 years he was to make several important contributions to knowledge. Chief among these was the work he completed at Cambridge developing what became known as the Hall Method for mapping and measuring minute concentrations of chemicals in biological specimens.
The shadow of suspicion never lifted from him, if suspicion is the right word for something that is known with certainty. Hall and his wife Joan, moreover, remained politically active in small ways, even becoming members of the Communist Party. When they moved to England, Hall told Albright and Kunstel, he was questioned by British intelligence along on the more- or-less open lines of: "We know what you have done in the past; why have you come here now?" He replied that he was a scientist who had been invited here to do scientific work, and that, it seems, was that.
His move from the United States in 1962 required US government approval, and that this was forthcoming remains curious. Why should the FBI, knowing what it did, have done Hall a favour? There is no evidence that he paid any price in terms of giving information, and he firmly denied this himself, but still it seems odd. It may be that they were simply glad to have him off their hands.
It was not until 1996, by which time he had retired and was already suffering from cancer and Parkinson's disease, that Hall's past caught up with him. The Venona papers had been declassified and opened to the public and (although it took some months before anyone noticed) his espionage record was there to be seen.
Exposed, eventually, in the press, he was denounced as a traitor and there were calls for him to be prosecuted, but the storm soon passed. He himself was absolutely unrepentant. In a statement reproduced in Bombshell, he wrote:
In 1944 I was 19 years old - immature, inexperienced and far too sure of myself. I recognise that I could easily have been wrong in my judgement of what was necessary, and that I was indeed mistaken about some things, in particular my view of the nature of the Soviet state. The world has moved on a lot since then, and certainly so have I. But in essence, from the perspective of my 71 years, I still think that brash youth had the right end of the stick. I am no longer that person, but I am by no means ashamed of him.
Ted Hall is survived by his wife Joan, who has known of his espionage record since before their marriage 52 years ago, and by two daughters, Ruth and Sara. A third daughter, Deborah, was killed in a road accident in 1992.
Theodore Alvin Holtzberg (Theodore Hall), physicist and spy: born New York 20 October 1925; married 1947 Joan Krakover (two daughters, and one daughter deceased); died Cambridge 1 November 1999.