Lt. Gen. Igor Kirillov, the head of Russia’s Nuclear, Biological and Chemical Defense Forces, and his assistant were killed during an explosion in Moscow on Tuesday.

Russia’s Investigative Committee said the explosive device was placed in a scooter near a residential apartment block, according to The Associated Press.

“Investigators, forensic experts and operational services are working at the scene,” RIC spokeswoman Svetlana Petrenko said in a statement. “Investigative and search activities are being carried out to establish all the circumstances around this crime.”

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