The French nuclear safety institute, which tracked the cloud, said that if the accident had occurred in France, the authorities would have taken measures to protect the local population within a few miles, and taken precautions over longer distances to halt the sale of contaminated crops. But the concentrations in the air over Europe, the institute said in a Nov. 9 report, “are of no consequence for human health and for the environment.”

Puzzlingly, on Oct. 9, regional authorities in the Chelyabinsk region, home of the plant, issued a statement saying that the Russian state nuclear corporation, Rosatom, had regularly tested the air and that “the radiation background in the region is within norms.”

A string of official denials followed. The press offices of several Russian nuclear plants issued statements denying any accidents or leaks and asserting that they had detected no elevated levels of ruthenium 106 in the air. One spokesman, for a plant in Smolensk, told RIA, a state news agency, “this is a rare element and we would have noticed it.”