A region in northern Russia has ordered more than 1,000 gas masks after a deadly explosion led to a spike in radiation, Russia’s Open Media news website reported.

At least five nuclear experts were killed in the explosion, which took place during a rocket test at sea off the coast of the Arkhangelsk region last Thursday. The incident has been cloaked in mystery, leading outside experts to theorize that the blast occurred during testing of a nuclear-powered cruise missile.