Mr. Karpov told a gathering of states that are party to the Comprehensive Nuclear Test Ban Treaty that the explosion did not involve a nuclear warhead, and thus did not fall within the treaty’s purview.

Instead, he said, the explosion was “linked to the development of weapons which we had to begin creating as one of the tit-for-tat measures in the wake of the United States’ withdrawal from the Antiballistic Missile Treaty.”

The Russian military initially reported that the Aug. 8 explosion near the village of Nenoksa, on the White Sea in northern Russia, released no contaminants — even as the local authorities at a city about 20 miles away reported a spike in radiation levels.

More misinformation and misdirection soon followed. In Moscow, some television channels blinked out on the night of the accident; screens turned blue and broadcast a warning for residents to remain indoors because of a storm with strong winds. Automated sensors that form part of an international network of radiation monitors, and which may have provided more clues about what happened, mysteriously stopped transmitting results.

Doctors and nurses at a hospital in the city of Arkhangelsk, about 40 miles from the blast, said they were not warned that patients arriving from the site were contaminated with radiation, and treated them without protective clothing.

Later, when one of the doctors was found to have a trace of radioactive cesium 137 in his tissue, the regional government in Arkhangelsk issued a news release saying the contamination was “with some degree of certainty” unrelated to the accident.

The doctor likely ate the element unwittingly in “fish, mushrooms, lichens, seaweed” or another food fouled earlier by fallout from some other, unspecified incident, the statement said.