Russia’s top security agency says it has arrested four Russians accused of making preparations to kill senior Defense Ministry officials on Ukraine's orders.

The arrest announcement follows the death of Russia's Lt. Gen Igor Kirillov, who was killed on Dec. 17 by a bomb hidden on an electric scooter parked outside his apartment building.

The bombs were disguised as an electric scooter, a document folder and a magnetized portable charger, Russia's Federal Security Service said.

According to the Federal Security Service (FSB), four Russians were involved in an alleged Ukrainian plot to assassinate senior military officers.