Call me naive, but I accepted to official answer that they were soviet jets that were either deliberately probing US air space (as someone said, to photograph missile bases) - hey, both sides did it repeatedly.. or genuine error (as the Soviet Government said) blown of course by error. They weren't exactly radioactive.. but carried 'low grade atomic weapons'
I think the point being that mutual distrust between the USSR / USA and them playing about with Nuclear weaponry ended up giving poor Japan all the problems.
PS edited in later.. If you read up on Wikipedia ( well known source of all things totally reliable) they sate USSR jet shot down.
Definitely Soviet. At least in the English version, the Soviet Ambassador ever admits they were Soviet planes when they are debating about who should take command of the mission.