In matters of nuclear threat, of course, European memories are framed by episodes such as the catastrophic accident in 1986 at the Chernobyl nuclear plant in Ukraine, then part of the Soviet Union; and, 20 years later, the lethal poisoning in London of a former K.G.B. officer, Alexander V. Litvinenko, with polonium 210, a rare and highly toxic isotope.

With such considerations in mind, the detection of elevated but unexplained levels of ruthenium 106 by monitoring stations in Austria, France, Germany, Italy and Switzerland raised alarms.

Slowly though, as the levels retreated, the fretting eased.

German and French nuclear security agencies concluded that the pollution had not threatened the health of Europeans or the environment in which they live. “These low levels of radioactivity do not pose a health hazard to the population,” the German radiation protection office said on Nov. 10.

The French institute reached similar conclusions, but said the amounts of ruthenium 106 released at the source of the accident were significant. If contamination of this magnitude had occurred in France, it said, the authorities would have ordered protective measures for people living miles around.

The nuclide, which does not occur naturally, is used for a variety of purposes, including as a therapy for cancers and as an energy source in satellites.

According to European scientists, the International Atomic Energy Agency, the United Nations’ nuclear watchdog, has investigated whether the pollution could have been caused by a satellite falling back to earth, but concluded that there had been no such event.

Nor, according to the French and German radiation protection agencies, could the pollution have been caused by an accident at a nuclear reactor, because only ruthenium 106 has been detected and such a spillage would have released other nuclides. That left two potential sources, the French agency said, “either in nuclear fuel-cycle facilities or radioactive source production.”