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.

Russia has arrested four people on suspicion of plotting to assassinate senior defence officials and their families on Ukraine’s orders, the Kremlin’s Federal Security Service (FSB) said.

Russian security forces have detained two suspects over the assassination of a top Kremlin general and his assistant in Moscow, Russian media reported on Dec. 18. Lieutenant General Igor Kirillov and his assistant were killed in Moscow on Dec.

Lieutenant General Igor Kirillov was the head of Russia's Nuclear, Biological, and Chemical Protection Troops. Authorities said the explosion was caused by a bomb hidden inside an electric scooter.

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

Lt. Gen. Igor Kirillov was killed Tuesday by a bomb hidden on an electric scooter outside his apartment building.

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.

Russia's security service said four Russian citizens were recruited by Ukraine to carry out the assassinations

The Federal Security Service of Russia said a series of assassination attempts had been foiled. Four Russian citizens have been arrested.