Russia has denied any of its power plants are behind a spike in radiation detected in the skies over northern Europe.
Scandinavian health authorities detected 'radionuclides' across the peninsula and in the Arctic Circle last week. They're not entirely sure where they came from, but said they are definitely artificial and not part of any natural phenomena.
"The composition of the nuclides may indicate damage to a fuel element in a nuclear power plant," the Dutch National Institute for Public Health and the Environment said.
Though an exact source can't be pinpointed, their analysis points to western Russia.
Lassina Zerbo, executive secretary of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), said its sensors also picked up the increased radioactivity levels, tweeting out a map showing the possible source locations - but adding it was "outside the CTBTO's mandate to identify the exact origin".
The marked part of the map covered parts of southern Sweden and Finland where there are nuclear reactors, but suspicion has fallen on Russia, which as the Soviet Union experienced the world's worst nuclear accident in 1986 at Chernobyl (now part of Ukraine).
In a statement, Rosenergoatom - a subsidiary of the Russia state nuclear agency Rosatom - said nothing was wrong with the two reactors it has in the region, both built in the 1970s.
"There have been no complaints about the equipment's work. Aggregated emissions of all specified isotopes in the above-mentioned period did not exceed the reference numbers. No incidents related to release of radionuclide outside containment structures have been reported.
"Radiation levels at both [plants] and surrounding areas remained unchanged in June, and no changes are also observed at present. It remains at levels that correspond to normal work of reactors. Those levels do not exceed natural background radiation figures."
The good news is health authorities in Scandinavia say the radiation levels, though unusual, aren't at this stage harmful to human health.
In 2017, a radioactive cloud passed over Europe, a study in 2019 concluding it probably came from Russia.
The Soviet Union in 1986 initially didn't tell the rest of the world about the disaster at Chernobyl, only admitting something had gone awry when the radiation released was detected in Sweden. Even then, they said the situation was under control - it wasn't, the fire burning in the reactor core for another couple of weeks.
The International Atomic Energy Agency on Saturday said it was aware of the radiation, and is seeking more information from affected countries.