The failed missile test that ended in an explosion killing five scientists last week on Russia’s White Sea involved a small nuclear reactor, according to a top official at the institute where they worked…

The blast occurred Aug. 8 during a test of a missile that used “isotope power sources” on an offshore platform in the Arkhangelsk region, close to the Arctic Circle, Russia’s state nuclear company Rosatom said over the weekend. The Defense Ministry initially reported two were killed in the accident, which it said involved testing of a liquid-fueled missile engine. The ministry didn’t mention the nuclear element.

It caused a brief spike in radiation in the nearby port city of Severodvinsk, according to a statement on the local administration’s website that was later removed. The Russian military said radiation levels were normal but disclosed few details about the incident.