In fall 2017, monitoring sites across Europe began to detect radioactive ruthenium-106 in the air. Analyses of the plume’s movement and its chemical makeup conducted since that time independently indicate that the leak came from the Mayak nuclear reprocessing facility in Russia. Russian authorities have refuted these findings. To more accurately pinpoint the origin of the plume, Olivier Saunier of the French Institute for Radiological Protection and Nuclear Safety (IRSN) and colleagues combined environmental measurements and atmospheric modeling (Proc. Natl. Acad. Sci.U.S.A. 2019, DOI: 10.1073/pnas.1907823116). The researchers found that a 250 TBq release of ruthenium-106 from the Mayak facility in the Ural Mountains on Sept. 28, 2017, best fits the data. Rosatom, the firm that runs the Mayak plant, has previously maintained that there have been no reportable events at any of its facilities during the period discussed. The firm also pointed out that an inspection of the Mayak facility after the 2017 measurements found no traces of an accident. C&EN asked Rosatom for a response to these new findings but did not receive a reply by the time this article went to press.