Few details have been known since 
five Russian scientists and two soldiers were killed earlier this month in an explosion at a military site in the White Sea's closed waters outside Severodvinsk.

Until now.

The blast was allegedly the result of a Burevestnik cruise missile test, according to experts at the James Martin Center for Nonproliferation Studies, a US research center at the Middlebury Institute of International Studies in Monterey, California.

The experts were among the first outside of Russia to find evidence of the radiation spike following the blast on August 8.

"Our operating theory is that there was a catastrophic failure of some kind during the testing of Russia's nuclear powered cruise missile. They call it Burevestnik but NATO refers to it as Skyfall," Anne Pellegrino, a research associate at the James Martin Center, told DW.

Putin's new weapon tested near Arkhangelsk?

Russian President Vladimir Putin first unveiled the Burevestnik cruise missile, which is powered by a nuclear reactor, during his state of the nation address in March 2018.

However, there has still been no official explanation in Russia as to which missile was being tested at the Nyonoksa test site, located in northwestern Russia's Arkhangelsk Oblast. Russian state nuclear agency Rosatom released a statement saying only that a "rocket engine" working from a "radioisotope power source" was being trialled. Some state-affiliated Russian media reported that a radioisotope thermoelectric generator (RTG), also known as a "nuclear battery," had been tested at the site.

In case of a failed test, an RTG would have emitted a much smaller amount of radioactive elements compared to the Burevestnik, according to experts at the James Martin Center.

Read more: Russia's nuclear-powered cruise missile, fact or fiction?

Satellite images reveal clues

US President Donald Trump earlier this month on Twitter attributed the "failed missile explosion in Russia" to the Skyfall missile.

James Martin Center experts agree, noting that the Nyonoksa test site was "in a state of disrepair, except for a single launch pad," which was modernized in 2018. Part of that modernization was a new structure on rails used for protection from radiation. Pellegrino pointed out that her colleagues had already seen this design at two other Burevestnik missile test locations — the Pankovo test facility on Novaya Zemlya and Kapustin Yar, both well-established Russian rocket launch and development sites.

Pellegrino said her colleagues used satellite imagery to determine that "the Russian military packed everything up" after testing on Novaya Zemlya, a remote archipelago in the Arctic Ocean, but they had no idea where the new tests would take place. After examining satellite images of Nyonoksa, they were able to quickly identify signs of the facilities needed for Burevestnik testing.

"When we looked at the satellite images, we saw that there was a barge or floating platform out off the coast of this test site," Pellegrino said.

The site is part of an exclusion zone in the White Sea, said Alexander Nikitin, a former Russian naval officer now working as nuclear safety expert at Bellona, an international environmental nongovernmental organization. "At the site, there are floating platforms used for all sorts of tests including this particular incident, when the rocket blew up," he explained.

Read more: Russia, China accuse US of stoking military tensions with missile test

Nuclear waste transporter spotted in White Sea

Experts at the James Martin Center noticed the presence of the nuclear waste transport ship Serebryanka in the area around the accident in Nyonoksa. That vessel, as well as other ships near the test site, were there for a particular purpose or had special permission, Pellegrino said. The Serebryanka had also been part of the recovery effort from the previous test that failed off the coast of Novaya Zemlya, she added.

Pellegrino believes preparations for the August 8 test lasted for around a year. She pointed out that the Serebryanka had been in the area well before the test had been conducted, suggesting the military knew a vessel of this sort would be required.

In the days following the blast in Nyonoksa, Norway recorded a radiation spike. The spike in itself is not necessarily a reason for concern, Pellegrino said, suggesting that it also could have been caused by a change in background levels of radiation, unconnected to the accident. However, she pointed out that the spike did support the theory that it was a mini nuclear reactor similar to the Burevestnik's being tested, rather than an RTG.