A Russian academic from the prestigious USA and Canada Research Institute of the Russian Academy of Sciences has been charged with treason.
Meanwhile, a visiting professor from the United States has had his papers confiscated by the Russian security police, apparently in connection with the same case.
Igor Sutyagin, who headed the institute's military-technical cooperation, was arrested "on suspicion" at the end of October.
He has since been charged under Article 5 (high treason) of the Russian criminal code.
The Russian federal security service has refused to give details of the charges in order to protect the "confidentiality of the investigation".
The Russian media noted, however, that Professor Sutyagin lived in Obninsk, a science town just outside the limits of Greater Moscow, which is the focus of Russia's nuclear research.
Institute director Sergey Rogov would not comment on the charges, saying that this was a matter for the courts to decide. He stated, however, that none of his staff worked with materials that could be construed to constitute state secrets.
Aleksey Yabolokov, a leading Russian ecologist, has his own views on the case.
He considers it an attempt, "and not the first", by the Russian security service to pressurise scientists who are dealing with problems of radiation safety and the environment.
He told the media that a few hours after Professor Sutyagin's arrest, security police raided the Moscow flat of Joseph Handler, a scholar from Princeton who has written extensively on radiation and nuclear safety and who was on a sabbatical at the institute.
His research materials, manuscripts, notebooks and a computer were confiscated, and he himself was questioned.