At least five nuclear experts have been killed in a mysterious explosion during a rocket engine test at sea in northern Russia four days ago.

Officials have been slow to release information about Thursday’s blast, which led to a radiation spike in a nearby city and sparked heightened demand for iodine.

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U.S.-based nuclear experts said they suspected the explosion occurred during the testing of a nuclear-powered cruise missile touted by President Vladimir Putin last year.

Here’s what we know about the explosion so far:

What happened?

— The rocket’s fuel caught fire, causing it to detonate and knock several people into the White Sea, the state nuclear agency Rosatom was quoted as saying by the state-run TASS news agency. 

— Officials in the nearby city of Severodvinsk said radiation had briefly spiked without saying how high, but their statement was taken down on Friday without explanation. Regional media reported that local residents had begun stocking up on iodine, which is used to reduce the effects of radiation exposure.