Officials reported that an unidentified assailant detonated an explosive device near a patrol vehicle in Moscow early Tuesday, resulting in his own death, the death of a police officer, and injuries to two other officers.

The attack happened minutes after midnight near the Savyolovsky Train Station in the Russian capital’s downtown, according to Moscow’s branch of the Interior Ministry, The Caspian Post reports, citing foreign media.

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 Investigative Committee said it has launched a probe into the attack. It didn’t name the assailant or give any information about his possible motives or any further details.

The attack came on the day marking the fourth anniversary of Russian President Vladimir Putin’s decision to send troops into Ukraine.