Russia's state nuclear agency has said five of its staff members were killed at a military testing site in northern Russia, reportedly when the liquid propellant rocket engine exploded during tests on a sea platform. Some reports say it may have involved a top secret weapon that was part of Moscow's hypersonic arsenal. Russia is pursuing hypersonic missiles as a nuclear deterrent, as Putin himself has recently verbalized. 

Other staff were being treated for serious burns after the accident; however, as Reuters reported, there's a run on iodine (used to reduce the effects of radiation exposure) in the northern port cities of Arkhangelsk and Severodvinsk, near where the mystery accident occurred.

"Everyone has been calling asking about iodine all day," one pharmacy was quoted as saying by a local media outlet in Arkhangelsk area, Reuters reported.

Thursday's mystery incident came two days after 16,500 people fled their homes when a separate immensely powerful series of blasts rocked an arms depot in Siberia, which had been caught in dramatic footage. 

And in a follow-up report to Thursday's mystery incident, Reuters pointed out initial emergency alert statements about a rise in radiation levels have been scrubbed from public record:

...authorities in the nearby city of Severodvinsk reported what they described as a brief spike in radiation. No official explanation has been given for why such an accident would cause radiation to spike.