The suspect, a 29-year-old citizen of Uzbekistan, is believed by investigators to have planted explosives outside the building where Kirillov and an aide were killed.

A statement by Russia’s Federal Security Service said the suspect had admitted under interrogation that he was recruited by Ukrainian intelligence.

On Tuesday, a source at the Security Service of Ukraine (SBU) claimed responsibility for the attack, describing Kirillov as an “absolutely legitimate target.”

Kirillov, who headed Russia’s Nuclear, Biological, Chemical Defense Forces, had been tried in absentia by the SBU on Monday on charges of “the mass use of banned chemical weapons" on the battlefield in Ukraine.

His assassination on a Moscow street was the most audacious in a series of killings of Russian officers believed to have been carried out by Ukraine’s intelligence services.