Russia’s main security agency said Thursday it had arrested several suspects accused of involvement in an alleged Ukrainian plot to kill senior military officers, a statement that follows the assassination of a top Russian general last week.

The Federal Security Service, the main successor to the KGB known by its Russian acronym FSB, said in a statement carried by Russian news agencies that it had arrested four Russians accused of plotting the assassination of senior Defense Ministry officials.

The FSB said the suspected masterminds of the attack planned to kill one of the senior officers using a remote-controlled car bomb. It is added that another high-ranking military official was to be killed by an explosive device hidden in an envelope. The agency did not name the military officers who were the target of the alleged plot.

The FSB released a video showing the arrest and interrogation of the suspects, who have not been named.

The statement followed the death of Lieutenant General Igor Kirillov, who was killed on December 17 by a bomb hidden on an electric scooter parked in front of his apartment building while he was on his way to his office. Kirillov’s aide was also killed in the brazen attack, which was claimed by Ukraine and brought the conflict back to the streets of the Russian capital.

FSB arrested the suspecta citizen of the Central Asian country of Uzbekistan, and claimed to have said he was recruited by Ukrainian special services.

Kirillov, 54, was the head of Russia’s radiation, biological and chemical defense forces. These special forces are tasked with protecting the military from the enemy’s use of nuclear, chemical or biological weapons and ensuring operations in a contaminated environment.

Russian President Vladimir Putin described the killing of Kirilov as a “big mistake” by Russian security agencies, stressing that they should learn from it and improve their effectiveness.

Meanwhile, Ukrainian officials said on Wednesday Russia launched a massive barrage of missiles and drones targeting Kiev’s energy infrastructure, attacked a thermal power plant and forced many to take shelter in metro shelters on Christmas morning.

Russia’s defense ministry said it carried out a “massive strike” on what it said were critical energy facilities in Ukraine that support the operation of Kiev’s military-industrial complex, Reuters reported.

The Russian attack left half a million people in the Kharkiv region without heating with temperatures just above zero degrees Celsius, and there were power outages in the capital Kiev and elsewhere, Reuters added.

At least six people were wounded in the strikes in Kharkiv and one was killed in the Dnipropetrovsk region, governors there said, Reuters reports.

In a statement to X, Ukrainian President Volodymyr Zelenskyy said more than 70 missiles, including ballistic missiles, and more than 100 attack drones were used to attack Ukrainian energy sources.

Putin deliberately chose Christmas to attack. What could be more inhumane?” Zelenskyy said. “They continue to fight for power outage in Ukraine.”

US President Biden on Wednesday condemned the Russian attack, saying that “the purpose of this outrageous attack was to cut off Ukrainians’ access to heating and electricity during the winter and to threaten the security of their grid.”