A mysterious explosion in far-northern Russia that killed seven people on Thursday is being linked by western nuclear experts to a failed test of a prototype nuclear-powered cruise missile.

Now, the weapon system once hailed by Russian President Vladimir Putin as “invincible” is suspected of causing one of the worst nuclear accidents in the former Soviet Union since the much bigger Chernobyl disaster of the 1980s.

“All the evidence points towards an explosion of a nuclear-powered cruise missile,” David Cullen, director of the UK’s Nuclear Information Service, told VICE News.

Russian officials have admitted five “national hero” scientists were killed testing a nuclear-powered engine, and reports of heightened radiation levels in the area have sparked a local run on iodine, which is used to treat exposure to radioactivity.

But the slow release of information by tight-lipped bureaucrats is raising concerns that the incident may have been even worse than officials have been willing to admit.

As details trickle in, here’s what we know about the accident, and about the missile system thought to have caused it, dubbed “SSC-X-9 Skyfall” by the U.S. and its NATO allies.

Mystery blast

Russia’s Department of Defense at first released only a brief statement saying two of its own employees had been killed testing a liquid propellant system.

But the country’s nuclear monopoly, Rosatom, admitted over the weekend that five of its staffers were also killed “during tests on a liquid propulsion system involving isotopes at a military facility.” At least three more people were hospitalized with injuries.

The blast occurred on an offshore platform in the region of Arkhangelsk near the Arctic circle, Rosatom said.

In this grab taken from a footage provided by the Russian State Atomic Energy Corporation ROSATOM press service, a Russian military band prepare to attend the funerals of five Russian nuclear engineers killed by a rocket explosion in Sarov, the closed city, located 370 kilometers (230 miles) east of Moscow, which has served as a base for Russia's nuclear weapons program since the late 1940s, Russia, Monday, Aug. 12, 2019. (Russian State Atomic Energy Corporation ROSATOM via AP)

Vyacheslav Solovyov, scientific director of the Russian Federal Nuclear Center, called the deaths “a bitter loss for our entire institute” in a video interview with a local newspaper, and said the researchers had been studying “small-scale sources of energy with the use of fissile materials.”

Alexei Likhachev, CEO of Rosatom, speaking at their funeral on Monday in the town of Sarov, said the men “died tragically while testing a new special device.”

The local city of Severodvinsk (pop. 185,000) announced a “short-term” spike in radiation levels immediately after the incident on Thursday. But the post on its municipal web-site was gone by the next day.

Greenpeace said data collected by Russia’s Emergencies Ministry showed a spike 20 times higher than normal levels in Severodvinsk, and demanded Russian authorities release more information about whether local residents face any threat to their health.

The neighboring country of Norway said it boosted its own radiation monitoring, but had so far not detected any increase in radiation on its own territory.

Skyfall: An unlimited-range missile

The most likely explanation appears to be that one of Russia’s new nuclear-fueled cruise missiles exploded on the launchpad, western nuclear experts said.

Putin himself unveiled the program in spring 2018, in the midst of his own campaign for reelection, to thunderous applause from a room full of Russia’s elite.

Designed to have “unlimited” range, the cruise missile propelled by an onboard miniature nuclear reactor could theoretically fly for days in circuitous patterns while hugging close to the earth’s surface, making it incredibly difficult to shoot down with existing missile defense systems.

Countries in the NATO alliance have dubbed it the “SSC-X-9 Skyfall.” Russia calls it the “Burevestnik,” or “Storm Petrel,” after the bird rumored by sailors to portend a coming storm.

Russia’s Ministry of Defense held an online “name-that-weapon” contest for the new missile last year, in which “Burevestnik” narrowly beat out another crowd favorite: “The Surprise.”

Nuclear propulsion would give the projectile a massive advantage in range when compared against a conventional liquid-fueled cruise missile, which can typically travel closer to 1,000 miles.

Putin himself showed a video clip of the missile zig-zagging around mountains and missile-defense systems, and veering around the southern tip of South America.

But arms experts remain skeptical that Russia can actually make such a complex system work.

U.S. skepticism

A U.S. intelligence report said last year that early tests revealed the supposedly “unlimited” range had capped out at a far less impressive distance of just 22 miles so far, and that four-out-of-four tests had crashed.

The American military once attempted its own version of a nuclear-powered cruise missile, but dropped the idea as too dangerous. That was partly due to a key design flaw: The engine spat out radioactive exhaust.

That meant such a missile could leave a dangerous radioactive plume behind, wherever it went on a long-winding flightpath.

“I suspect the Russians are walking into a bit of a developmental dead end,” said Ankit Panda, an Adjunct Senior Fellow in the Defense Posture Project at the Federation of American Scientists. “The United States tried to get nuclear propulsion for cruise missiles working and we quickly discovered the immense pitfalls.”

Thursday’s incident is just the latest in a series of high-profile Russian military accidents.

Days earlier, a Russian military depot packed with 40,000 shells spectacularly exploded in the Siberian town of Achinsk, prompting the evacuation of more than 10,000 people.

In July, a fire broke out on a top-secret Russian submarine believed to be involved in underwater espionage activities called the “Losharik,” killing 14 sailors.