Nov. 22 (UPI) -- Russian scientists killed in a nuclear explosion at a secret military testing site in August were working on "unparalleled" weapons, Russian President Vladimir Putin said.
"We're talking about the most advanced and unparalleled technical ideas and solutions, about weapons designed to ensure Russia's sovereignty and security for decades to come," Putin said during a ceremony at the Kremlin on Thursday honoring the victims of the mysterious accident.
The Aug. 8 explosion of a liquid propulsion system in Nyonoksa killed five scientists and two defense ministry officials and led to international concern about a brief spike in radiation levels reported by the Russian meteorological agency Rosgidomet.
"Tests of new weapon systems were involved. For this reason it is only natural that the investigation being carried out in this connection cannot be public," Peskov said.
U.S. officials previously said they believed the incident involved a nuclear-powered cruise missile Putin said was under development last year.
Putin presented posthumous orders of courage awards Thursday to the widows of the scientists killed in the blast.
"They were working on a very difficult, important and critical assignment," Putin said at the ceremony. "We're talking about the most advanced and cutting-edge technical ideas and solutions, unequaled anywhere in the world; the weapon that is entitled to ensure sovereignty, security of Russia for decades to come."