An “exceptionally high amount” of radioactivity was detected over Finland's capital on March 3 and 4.

Officials from the Radiation and Nuclear Safety Authority ( STUK ) have said their rooftop air sampler in Helsinki detected 4,000 microbecquerels of the radioactive isotope caesium-137 per cubic meter of air. This is around 1,000 times the regular amount they detect.

Oddly, radiation levels on the following two days returned to 12 microbecquerels per cubic meter of air, which is declared a normal level.

Caesium-137 is used for medical devices, industrial processes and in research centers. Although large concentrated exposure to the isotope can cause radiation sickness and even death, the authorities said that levels were never high enough to affect human health.

Radiation is regularly detected above Helsinki, which the STUK says drifts from the Chernobyl nuclear power plant fallout over 1,000 kilometers (620 miles) away. However, the fact that the sample only contains caesium-137 indicates these exceptionally high levels aren’t linked to any form of nuclear reactor.

For a number of days, the source remained a mystery. On March 8, the STUK released an update that said they had traced the source back to their own building's garage. Their property complex also houses a small radioactive waste treatment facility. The facility has since been isolated, with further investigations underway.