Soviet “illegals” were spies operating under deep cover, without diplomatic (“legal”) protection. They were out in the cold, not attached to a Soviet embassy. This meant that, if caught, they could be arrested because they did not have diplomatic immunity.
Even now, eight decades later, secrets about Soviet spies in the MANHATTAN Project, the Anglo-American wartime nuclear bomb project, are still being disclosed.
One of the most elusive such spies is Arthur Adams. Adams seems to have been born in Sweden, but moved to tsarist Russia, where he was persecuted, leading to his emigration to the United States. According to some accounts, he served in the US army during the World War I. After the war, he travelled between the US and the Soviet Union, while employed by a Soviet trade organization in the US, AMTORG, which was in reality a Soviet espionage front group. Before the World War II, Adams was made the Soviet military intelligence (GRU) illegal chief of station (“rezident”) in the United States (working under the codename ACHILLES).
Wartime atomic espionage became a key target for Adams’ spy network thereafter. Adams was one of the first Soviet agents to detect the existence of the MANHATTAN project. One of his agents was codenamed ESKULAP, whose identity remains unknown in the West. Another of Adams’ agents was a scientist named Clarence Hiskey (codename RAMSAY), who worked in a branch of the MANHATTAN project at Chicago University. In total, from his agents Adams managed to obtain over 5,000 classified documents from the MANHATTAN project, and samples of radioactive materials.
By November 1944, the FBI was onto Adams, not least because he was observed getting into the car of a Soviet official in New York, who was in fact the GRU legal resident. As the net closed on Adams, however, he managed to elude the FBI and later escaped to the Soviet Union. US authorities got rid of Hiskey by sending him to the Arctic, where he was given the job of counting winter underwear.
A clue about how Adams managed to disappear may lie in his now-declassified MI5 dossier. In 1946, a key British report about Adams was written by an MI6 agent who in fact was a highly successful Soviet spy, Kim Philby. The FBI had previously informed British intelligence about its investigation into Adams. It is thus possible that Philby, in MI6, managed to discover this, and, through his Soviet handlers, passed a warning to Adams—allowing him to concoct an escape. Years later, after the Soviet Union collapsed, president Boris Yeltsin of Russia would honor Adams posthumously, naming him a Hero of the Russian Federation.