Five nuclear engineers were killed in a liquid propulsion system blast at Russia’s naval missile test facility, leading to a brief spike in radiation on Aug. 8. The secrecy surrounding the accident has led outside observers to speculate that what the explosion involved the Burevestnik nuclear-powered intercontinental cruise missile, dubbed the SSC-X-9 Skyfall by NATO.

The mysterious explosion in northern Russia that caused a spike in radiation levels happened during a mission to salvage a nuclear-powered cruise missile from the bottom of the sea, media have cited a U.S. intelligence assessment as saying.

“This was not a new launch of the weapon, instead it was a recovery mission to salvage a lost missile from a previous test,” the CNBC business outlet cited an unnamed U.S. official with direct knowledge of a U.S. intelligence assessment as saying.

Russian crews aboard three vessels had last year prepared to recover a missile that landed in the Barents Sea during a failed November 2017 test, CNBC reported last year, also citing a U.S. intelligence report.

“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 lead to the radiation leak,” another unnamed source told the outlet Thursday.

The U.S.-funded Radio Free Europe/Radio Liberty news outlet’s Russian-language service came to the same conclusion after analyzing photographs of nuclear waste containers at what are thought to be previous Burevestnik test sites.

Russia tested four of the missiles between November 2017 and February 2018, each resulting in a crash, people who spoke on condition of anonymity previously told CNBC.

Government officials have given a muted, occasionally contradictory response in the weeks since the accident.

President Vladimir Putin said the explosion occurred during testing of what he called promising new weapons systems. Last year, Putin had boasted about what he said was the Burevestnik’s unlimited range.

Four of Russia’s nuclear radiation monitoring stations went silent days after the explosion, and doctors in the region have said they weren’t warned that they were treating patients exposed to radiation.