A U.S. intelligence assessment found that the mysterious explosion off of Russia’s northern coast occurred during a recovery mission to salvage a nuclear-powered missile from the ocean floor, according to people with direct knowledge of the report.

The mysterious explosion on Aug 8 killed five scientists and sparked fears that Russia had tested its new nuclear-powered Burevestnik missile, also known as Skyfall, CNBC recalls.

"This was not a new launch of the weapon, instead it was a recovery mission to salvage a lost missile from a previous test," said a person with direct knowledge of the U.S. intelligence assessment. "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," said another person, who spoke to CNBC on the condition of anonymity.

The U.S. intelligence report did not mention potential health or environmental risks posed by damage to the missile's nuclear reactor.

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CNBC learned last year of Moscow's similar preparations to try to recover a nuclear-powered missile lost at sea. Crews attempted to recover a missile that landed in the Barents Sea after a failed test. The operation included three vessels, one of which is equipped to handle radioactive material from the weapon's nuclear core. If the Russians were able to regain possession of the missile, U.S. intelligence analysts expected that Moscow will use the procedure as a blueprint for future recovery operations.

Last March, Russian President Vladimir Putin unveiled a slew of hypersonic weapons, as well as Burevestnik, saying it was a nuclear-powered missile with unlimited range. However, the Kremlin has yet to perform a successful test of the weapon over multiple attempts, according to sources with direct knowledge of a U.S. intelligence report on the weapons program.