The first soviet atomic bomb was detonated in 1949[1] so this was nearly 10 years after that fact.
His association to Klaus Fuchs[2] was probably more suspect than Oppenheimer (despite how suspicious they were of him) given he was allegedly an actual spy (I say allegedly because I know how suspicious of the FBI people are around here, and not without reason in some cases).
According to the Feynman Bio I'm reading right now [3], Fuchs was said to have joked (when Feynman wasn't around) that he would be the most likely person to be a spy because of how often he left the base alone to visit his sick wife (or so he said).
"I say allegedly because I know how suspicious of the FBI people are around here"
Fuch's was caught by MI5, not the FBI. And he gave a detailed confession even before he was arrested. So there isn't really any reason to be suspicious of his conviction.
Feynman himself casually notes Fuchs as "the fellow who turned out the be the spy", and despite their friendship never offered an skepticism about the conviction.
I'm not familiar with this particular case, but innocent people confess more frequently than you might think. Investigators put tremendous pressure on suspects during interrogation, and they can often feed details of the case to the suspect, resulting in a 'detailed confession'. Many times, the investigator is not intentionally trying to get a false confession.
That's not the case here and you can simply google the guy, his case is famous. He confessed voluntarily, likely because he was tipped off and knew he was already exposed. After serving his time he continued his life in East Germany. There is no doubt whatsoever he was a spy and he never disputed it when no longer under duress.
Good catch, I almost looked it up but I got lazy. I don't know if the Feynman bio I'm reading specified exactly who caught him but it didn't seem like there was ever any particular question whether or not he had done it but I can't either say that I know he was, I've just read it and not looked into the details myself.
It doesn't matter if Fuchs is an actual spy for the purposes of figuring out if the FBI should be worried - all that matters is that the FBI believes that he's a spy.
His association to Klaus Fuchs[2] was probably more suspect than Oppenheimer (despite how suspicious they were of him) given he was allegedly an actual spy (I say allegedly because I know how suspicious of the FBI people are around here, and not without reason in some cases).
According to the Feynman Bio I'm reading right now [3], Fuchs was said to have joked (when Feynman wasn't around) that he would be the most likely person to be a spy because of how often he left the base alone to visit his sick wife (or so he said).
[1] http://www.atomicheritage.org/history/soviet-atomic-program-... [2] https://en.wikipedia.org/wiki/Klaus_Fuchs [3] https://www.amazon.com/Genius-Life-Science-Richard-Feynman/d...