Update: An Aug. 13 report by Russian news agency Tass suggests an evacuation of Nyonoksa will no longer take place.

MOSCOW — At around 6 a.m. GMT on Aug. 8, seismic and acoustic sensors in Sweden, Finland and Norway detected an explosion. The sensors are operated by the Comprehensive Nuclear-Test-Ban Treaty Organization, and on Monday the group — which monitors the globe for prohibited nuclear tests — said four stations identified an event “coinciding” with an explosion in Nyonoksa, Russia.

The organization did not share any data about the size and nature of the explosion. And though little is known about what happened on Russia’s northern frontier last week, it seems safe to say there was no detonation of a nuclear weapon.

But that does not make the Nyonoksa incident any less concerning.

In the wake of the explosion, the city government of Severodvinsk — a major military shipbuilding town about 40 kilometers from the explosion — said local monitors detected a brief spike in radiation levels. The military was quick to deny, asserting no harmful materials were released into the air. Pharmacies in Severodvinsk reportedly saw a run on iodine tablets.

A statement issued Sunday by the Norwegian Radiation and Nuclear Safety Authority, an outfit that spends a lot of its time standing watch for undisclosed Russian radiation hazards, said Norwegian monitoring stations detected no such increase. Neither did European or Russian stations to which the agency has access.

Initial reports in the Russian press were confusing and contradictory, and at least some of them were perhaps deliberate fakes. At first, the precise location of the explosion was unclear. Did it happen at sea, as the Tass news agency reported? Or perhaps at the Russian Navy’s missile test range at Nyonoksa, as other, more reputable outlets reported?

Whatever was detected in Severodvinsk was highly localized. Or perhaps never happened all. The city administration quietly withdrew its claim of a brief radiation spike over the weekend. This has only heightened concerns that the military is covering something up.

Unconfirmed reports Tuesday suggested Nyonoksa residents were being prepared for evacuation. However, Tass published a story later quoting the head of the village as saying he got a call from the test range commander, who said all of their planned activities have been canceled. The head of the town then said there’s no need for an evacuation and that residents should proceed as usual.

Unconfirmed videos showing medical personnel responding to the incident in hazmat suits surfaced on social media on Aug. 8. As did videos showing ambulances shielded with plastic tarps transporting the wounded to a Moscow hospital to be treated for radiation burns. In Russia and in the West, concerns quickly grew of a small-scale Chernobyl incident.

The Russian military, for its part, was not helpful. In an Aug. 8 statement, the military said the explosion took place at a military testing ground. As for what exploded, it only referenced a test of a liquid-fuel propulsion system.

Initial speculation within the Russian press suggested several possibilities based on what little evidence was available. One of those included a failed test of a missile already in service with the Russian Navy, perhaps something using a highly toxic fuel known as heptyl. Another story, based on unidentified sources, claimed it was Russia’s new Tsirkon anti-ship hypersonic missile.

But the detection of elevated radiation levels in Severodvinsk has been hard to ignore, and consensus has settled on a third theory: a failed test of one of President Vladimir Putin’s nuclear vengeance weapons announced in a saber-rattling speech last year. Specifically, the 9M730 Burevestnik nuclear-powered cruise missile, known to NATO as SSC-X-9 Skyfall.

A failed test of Burevestnik would explain the radiation spike, the apparent secrecy, the hazmat response team and the extension on Aug. 8 of a sea lane closure in the region around the Russian military’s Nyonoksa missile test range. Western experts have since compiled additional, compelling evidence that the device in question was indeed Burevestnik.

In a thread on Twitter, arms control expert Jeffrey Lewis with the Center for Nonproliferation Studies laid out the case for Burevestnik: Commercial satellite imagery of the Nyonoksa range suggests the site has been modified to resemble a remote range at Novaya Zemlya where Western experts believe earlier Burevestnik tests were conducted.

In the same photo of the Nyonoksa, taken by Planet Labs on Aug. 8, Lewis and his team identified a nuclear fuel-carrying ship that has shown up in suspected Burevestnik tests at Novaya Zemlya. A report by The New York Times on Monday featured other Western experts and officials signing on to the Burevestnik theory.

But the simple fact is that almost nothing is known about Burevestnik. And it is entirely possible the entire program, perhaps a real budget item, is doomed to fail. Its real purpose is likely to coax the U.S. into talks on future arms control treaties that limit its missile defense ambitions.

The entire point of Burevestnik is unlimited range to maneuver around missile defenses. Over the past year, a basic understanding of the Burevestnik design has been adopted by observers, essentially by default. The U.S. in the 1950s and 1960s experimented with a nuclear ramjet engine called Project Pluto. The entire concept was scrapped, both for being a bad idea and for being too large to fit into a cruise missile design.

The assumption has been that Burevestnik is a nuclear ramjet. But such a concept does not square nicely with official descriptions of the device in question issued over the weekend by Rosatom, Russia’s state nuclear concern.

On Saturday, an official statement from Rosatom confirmed five of its specialists died in the blast, and identified them as employees of the All-Russian Scientific Research Institute of Experimental Physics (VNIIEF, by its Russian acronym). This institute is Russia’s equivalent of the United States’ Los Alamos National Lab, and it’s Russia’s premier nuclear research facility where the Soviet atomic bomb was born.

VNIIEF was also identified last year by newspaper Kommersant as the design house for the Burevestnik propulsion system.

In its statement, Rosatom explained that the explosion took place during the test of an “isotopic power source” within or mounted to a “liquid propulsion system,” depending on how you interpret the Russian-language phrasing. Either way, this is not the description of a ramjet system — which would work by a nuclear reactor heating air as it passes through an intake.

On Sunday, the scientific director of VNIIEF, Vyacheslav Solovyov, issued a statement on camera in which he described work on miniature nuclear power devices, such as new kinds of radioisotope thermoelectric generators or small nuclear reactors like NASA’s Kilopower project. The implication was that this is what the specialists were working on.

Neither Rosatom nor Solovyov gave any other indication that a missile was involved.

Taken at face value, the Rosatom statements suggest at least two possibilities: The Burevestnik uses some other form of nuclear propulsion other than a nuclear ramjet, or that the Nyonoksa incident involved something other than the Burevestnik. And given the scope of current Russian nuclear efforts, this possibility should be seriously entertained.

If Burevestnik is not powered by a ramjet, then perhaps the vague descriptions issued by Rosatom point to a different — perhaps even more risky — form of propulsion. This could be a nuclear thermal rocket, which sees liquid fuel pushed through a reactor. But why, then, would testing be moved from Novaya Zemlya, as Lewis claims, and closer to population centers?

A likely candidate for an alternate explanation would be Poseidon, announced by Putin last year alongside Burevestnik. Poseidon is an underwater drone packed with a large nuclear bomb. It is intended to crawl up to a coastline and detonate, causing massive damage. An another possible explanation is an underwater atomic battery for Russian seafloor infrastructure.

Another scenario, according to Russian aerospace analyst Pavel Luzin, is that the test involved — as Rosatom suggested — a of new kind of radioisotope thermoelectric generator or a small reactor for use in spacecraft. Luzin speculates that Russia may have been testing the impact of rocket engine vibrations on the new power source.

Ultimately, the problem with the Nyonoksa incident is that it’s unclear what anyone, on any side, is actually talking about. And attempts to connect the dots between certain observations and vague official statements can lead one to reasonably argue several plausible scenarios.

If the evacuation were still on, it would have at least suggested the Russian government was ready to continue testing whatever exploded last week.

Matthew Bodner covered Russian affairs for Defense News.