A US State Department report submitted to the UN last week provided more details about a nuclear accident in Russia in August and killed seven Russian engineers.

The report's author told Insider that Russia's reaction to the accident, which occurred during attempts to recover a nuclear-powered cruise missile from the bottom of the sea, amounted to a cover-up.

Another expert told Insider that radioactive materials could still be present in the surrounding area, putting inhabitants in danger.

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Russia's handling of a nuclear accident in August amounts to a cover-up, according to a State Department official who investigated the incident and reported the findings to the UN last week.

In an interview, Thomas DiNanno, deputy assistant secretary and senior bureau official at the State Department's Bureau of Arms Control, Verification, and Compliance, pointed to the difference between his committee's findings about the incident and Russia's repeated denials and obfuscations.

Signs point to a cover-up

When Insider asked whether his department's findings pointed to a cover-up, DiNanno said, "Yes, that's exactly what we're saying."

DiNanno's report found that on August 8, an accident occurred while Russian engineers attempted to recover a nuclear-powered cruise missile from the bottom of the White Sea. The missile had been there since a failed test in February 2018.

After the explosion, Russian officials called for an evacuation of an area near Nyonoksa, Russia, and then called it off.

Four nuclear monitoring stations went offline shortly after the accident, and Russian officials did not tell doctors treating victims of the accident about potential radiation poisoning. Later, Russian security agents requested that hospital staff sign non-disclosure agreements, The Moscow Times originally reported.

An entrance checkpoint to a military garrison near the village of Nyonoksa in Arkhangelsk Region, Russia, October 7, 2018. Reuters

"One of the things we were clear about is Russia has to answer for this — they have a lot to answer for," not just to the Russian people and nearby countries but to the entire world, DiNanno said.

"This affects countries in South America [and] Africa as much as it does those immediate countries. It's not specifically about the accident. It's about how reckless Russia's weapons development program is, how aggressive it is," he said.

The weapon, the 9M730 Burevestnik, a nuclear-powered cruise missile also called Skyfall, is part of what DiNanno referred to as an exotic weapons package. The Skyfall missile is not covered by the new START treaty, a nuclear nonproliferation treaty between the US and Russia.

"We believe these exotic systems either shouldn't exist, and if they do exist, we have to have a discussion on ... how they would be controlled," DiNanno told Insider. The Skyfall missile is one of five so-called exotic systems Russia announced last year that are not authorized under the treaty.

DiNanno estimated that the missile, which was not armed with nuclear warheads, was between about 60 miles and 120 miles from a population center while it was the bottom of the White Sea after the failed test last year.

But when the weapon's nuclear reactor exploded during recovery efforts, "The Russian reaction to this was immediately to cover it up and shut off the international monitoring stations in Russia that are part of the CTBT," or Comprehensive Nuclear Test-Ban Treaty, although the treaty is not yet entered into force, DiNanno said.

'This behavior is not acceptable'

A street in the military garrison near the village of Nyonoksa in Arkhangelsk Region, Russia, October 7, 2018. The board reads: "State Central Naval Range." Reuters

An expert with the State Department told Insider that the explosion still posed a danger to those in the vicinity.

"A serious decontamination effort should be going on as we speak," the expert said.

There also needs to be some effort to determine how far the fission products — the results of the nuclear explosion, which are radioactive — have permeated in the area.

"As long as they stay there, their half-lives are in hundreds of years, and they should be cleaned up," the expert said.

"We chose the venue of the UN to talk about this for a reason," DiNanno told Insider. "Maintaining these weapons is an awesome responsibility. This behavior is not acceptable."