A blast in Moscow kills 2, including the founder of a unit fighting in eastern Ukraine, reports say

An explosion rocked an upscale residential block in Moscow early Monday, killing two people – including the founder of a battalion that battled Kyiv’s forces in eastern Ukraine -- and injuring three others, Russian authorities and news reports said.

Footage released by Russia's Investigative Committee showed a building lobby with shattered glass doors and a damaged ceiling. It didn't say what caused the blast, but the state news agency Tass cited an unidentified law enforcement source as saying a bomb was planted in the building.

The Investigative Committee said it opened a criminal inquiry on charges of murder by means injurious to the public, attempted murder of two or more people and arms trafficking.

Tass and the RIA Novosti news agency, both citing a law enforcement source, identified one of the dead as Armen Sarkisyan, the founder of a volunteer battalion fighting in Ukraine who also heads a boxing federation in the Russian-controlled Donetsk region. Both agencies said he died in the hospital.

Tass also reported that Sarkisyan’s bodyguard was killed by the blast.

Officials initially reported that the blast killed one and injured four, but later said one of them died in the hospital. Three others remain hospitalized.

In December, Sarkisyan was charged in absentia by the Security Service of Ukraine with participation in illegal armed or paramilitary formations or assisting such formations in combat operations against its armed forces.

The agency, known as the SBU, has previously accused Sarkisyan of recruiting prisoners to fight in Ukraine, cooperating with Russian security forces, and creating an armed formation involved known as “ArBAT,” or “Armenian Battalion”.

The ArBAT group fought the Ukrainian army in the Toretsk area of the Donetsk region, as well as in Russia's Kursk region, the SBU said. Ukrainian forces launched an incursion in the Kursk region in August.

The Russian authorities did not mention Ukraine in their statements about the explosion, and the Ukrainian authorities also have not yet commented.