A small nuclear reactor was involved in the failed missile test last week in Russia that ended in an explosion that killed five scientists, Bloomberg News reported Monday.

The institute they worked for is developing small-scale power sources that use “radioactive materials, including fissile and radioisotope materials” for the Defense Ministry and civilian uses, Vyacheslav Soloviev, scientific director of the institute, said on local TV, according to the news outlet.

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The scientists were national heroes and the “elite of the Russian Federal Nuclear Center,” institute Director Valentin Kostyukov said in the video obtained by Bloomberg.

The blast, which occurred Aug. 8, caused a brief spike in radiation in the nearby port city of Severodvinsk, according to a now-deleted statement on the local administration’s website reviewed by Bloomberg.

Russian media have speculated that the test involved the SSC-X-9 Skyfall, known in Russia as the Burevestnik, a nuclear-powered cruise missile.

Russian President Vladimir Putin Vladimir Vladimirovich PutinPutin nominated for Nobel Peace Prize Navalny released from hospital after suspected poisoning Ex-Trump national security adviser says US leaders 'making it easy for Putin' to meddle MORE introduced the weapon during his state-of-the-nation address last year.