Russia's nuclear agency said Saturday that five of its employees were killed earlier this week when a rocket engine exploded during a missile test at a military base in the far northern Arkhangelsk region.

The explosion led to radiation levels spiking up to 20 times above normal in the nearby city of Severodvinsk for about a half an hour, according to the Guardian. The defense ministry initially reported that no radiation change was detected, but officials in Severodvinsk later contradicted those claims in a statement that was taken down on Friday without explanation.