US intelligence suspects that a mysterious explosion at the Nyonoksa testing range earlier this month occurred during Russian efforts to recover a nuclear-powered cruise missile lost at sea during a previous test, CNBC reported on Thursday, citing two sources with direct knowledge of the latest assessment.

The August 8 blast killed at least five and triggered a slight radiation spike in nearby towns. Russian data on the incident suggested a nuclear reactor was involved, but Russia has not acknowledged that it was a nuclear accident.

"There was an explosion on one of the vessels involved in the recovery and that caused a reaction in the missile's nuclear core which led to the radiation leak," one source told CNBC.

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US intelligence suspects that a mysterious and deadly explosion earlier this month was caused by Russia's efforts to recover its new nuclear-powered cruise missile after another unsuccessful test, CNBC reported on Thursday, adding another twist in the saga of what exactly happened at the Nyonoksa weapons testing range.

An explosion on August 8 that killed at least five people and triggered a radiation spike in nearby towns has been linked to Russia's development of the 9M730 Burevestnik nuclear-powered cruise missile, a new doomsday weapon that NATO calls the SSC-X-9 Skyfall. While the prevailing theory was that the blast was caused by a failed test, US intelligence has a slightly different explanation.

"This was not a new launch of the weapon, instead it was a recovery mission to salvage a lost missile from a previous test," a source with direct knowledge of the latest intel reports told CNBC. Russia was said to be salvaging the weapon from the ocean floor at the time of the incident.

"There was an explosion on one of the vessels involved in the recovery and that caused a reaction in the missile's nuclear core which led to the radiation leak," another source told CNBC.

This is not the first time Russia has had to go fishing for its nuclear-powered cruise missile, but this appears to be the first time a recovery effort has exploded.

Using nuclear reactors to fuel missiles or airplanes has proved to be a "hazardous" technology that's probably unnecessary, a leading defense expert told Insider this week.

Read more: Putin lost his supposedly 'invulnerable' nuclear-powered missile at sea — now he has to go find it

Russia has not been particularly forthcoming with the details, sparking concerns about a cover-up.

The death toll has risen from two to five, and it could be higher. Russia has flip-flopped on acknowledging radiation leaks. Local authorities ordered an evacuation but then mysteriously canceled it. Nuclear-monitoring stations nearby unexpectedly went offline because of technical problems. And the system that triggered the explosion has been described as everything but the nuclear-powered cruise missile that Russian President Vladimir Putin last year boasted would be unstoppable.

Read more: It looks like the Russians are trying to hide the truth about that nuclear accident in Nyonoksa

"This is work in the military field, work on promising weapons systems," Putin said recently, adding that "when it comes to activities of a military nature, there are certain restrictions on access to information."

Read more: New details on Russia's mysterious missile disaster suggest a nuclear reactor blew up

Russian data on the brief radiation spike in Severodvinsk — which state authorities finally released this week — indicated that a nuclear reactor was involved, experts said. Russia, which has a history of covering up nuclear disasters, has not acknowledged that it was a nuclear accident, despite mounting evidence to the contrary.