• Russia's head of chemical weapons was killed on Tuesday when a scooter bomb exploded in Moscow.
• Ukraine was behind the attack, a Ukrainian Security Service source told BI.
• Igor Kirillov is the most prominent military official to be killed since Russia invaded Ukraine.

A high-ranking Russian general responsible for Russia's chemical weapons was killed on Tuesday by a bomb placed in a scooter on a Moscow street.

A source inside Ukraine's Security Service with knowledge of the attack told Business Insider the agency was behind the death of Lieutenant General Igor Kirillov.

The Wall Street Journal also reported Ukrainian officials as saying the killing was a special operation by the Security Service of Ukraine.

Kirillov, the head of Russia's Nuclear, Biological, and Chemical Protection Troops, was killed by a bomb planted in a scooter parked on a street in Moscow, Russia's investigative committee said in a statement on Telegram on Tuesday.

"According to the investigation, on the morning of December 17, an explosive device was detonated in a scooter parked next to the entrance of a residential building on Ryazansky Prospekt in Moscow," the statement said.

"As a result of the incident, the head of the radiation, chemical and biological protection troops of the Armed Forces of the Russian Federation Igor Kirillov and his assistant were killed," it added.

The committee said it had opened a criminal case and that investigators and forensic experts were working at the scene.

The Russian investigations committee didn't immediately respond to a request for comment from BI.

According to the Ukrainian source, the bomb on the scooter was detonated when Kirillov and his assistant were entering a nearby house on Ryazansky Prospekt. BI couldn't independently verify the claim.

"Kirillov was a war criminal and an absolutely legitimate target," they said, accusing Kirillov of giving orders to use banned chemical weapons against Ukrainian forces.

"Such an inglorious end awaits all those who kill Ukrainians. Retribution for war crimes is inevitable," they added.

Kirillov is the most prominent military official to be killed since Russia launched its full-scale invasion of Ukraine in 2022, according to the Financial Times.

He was sanctioned by the UK in October for the use of chemical weapons in Ukraine, including the choking agent chloropicrin.

Kirillov's death follows a string of similar attacks, some of which sources said were the work of Ukraine's Security Service or other agencies.

Dmitry Medvedev, deputy chair of Russia's Security Council and a former Russian president, described Kirillov's killing as a terrorist attack, and offered his condolences to Kirillov's family, per the TASS news agency.

He also said that Ukraine would pay for its actions.

This is a developing story. Check back for updates.