MOSCOW (AP) — An unidentified assailant detonated an explosive device next to a patrol vehicle in Moscow early Tuesday, killing himself and a police officer, and leaving two other officers injured, officials said.

The attack happened minutes after midnight near the Savyolovsky railway station in the Russian capital’s downtown, according to Moscow’s branch of the Interior Ministry.

It said the assailant approached a traffic police vehicle and detonated an explosive device, killing an officer on the spot and injuring two others, who were hospitalized.

Russia’s top investigative agency later said the assailant was a 22-year-old man from Russia’s Udmurt Republic, more than 1,100 kilometers east of Moscow. It said that a probe was ongoing into the motives behind the attack.

Speaking at a meeting with top officials from Russia’s Federal Security Service, or FSB, Russian President Vladimir Putin said that the man had been recruited online and given the explosive device unknowingly. It was then detonated remotely, he said.

The attack came four years after Russia sent troops into Ukraine.