A global monitoring network this week detected unexplained "higher than usual levels" of radioactive isotopes at a station in Sweden that likely came from somewhere around the Baltic Sea.
But the group also said late on June 26 that the levels it saw were "not harmful for human health."
The head of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), Lassina Zerbo, tweeted that the elevated levels of three radionuclides generally associated with civil nuclear activities -- cesium-134, cesium-137, and ruthenium-103 -- had been detected on June 22-23.
He highlighted an area including stretches of western Russia, seemingly including St. Petersburg, Estonia, southern Finland, and Sweden as a "possible source" of the radioactive particles at any point in the preceding 72 hours.
The Finnish, Norwegian, and Swedish nuclear safety agencies also said they'd identified amounts of radioactive isotopes in the area, adding the levels did not appear harmful to humans.
The Dutch National Institute for Public Health and the Environment said on June 26 that its "calculations show that the radionuclides come from the direction of western Russia."
Their character "may indicate damage to a fuel element in a nuclear power plant," the Dutch said.
But Russia's hydrometeorological and environmental monitoring agency, Roshydromet, said on June 27 that it had detected no increase in radioactive particles.
Russia has several operational nuclear power plants in its northwest, including near Kola, St. Petersburg, and Smolensk.
The CTBTO oversees hundreds of monitoring stations that use seismic, acoustic, and other technology to look out for weapons testing anywhere in the world.
But the network can also spot other anomalies.
Radioactive particles can be carried long distances by the wind.
"These are certainly nuclear fission products, most likely from a civil source," a spokeswoman for the Vienna-based CTBTO said, according to Reuters.
That would suggest the particles originated from the atomic chain reaction that powers a nuclear reactor.
"We are able to indicate the likely region of the source, but it's outside the CTBTO's mandate to identify the exact origin," the source added.
The Swedish Radiation Safety Authority said on June 23 that it was "not possible now to confirm what could be the source of the increased levels" of radioactivity.
"The radionuclides are artificial, that is to say they are man-made," the Dutch safety watchdog said. But "a specific source location cannot be identified due to the limited number of measurements."