MOSCOW - The four individuals directly involved in the Moscow concert hall attack have been charged with terrorism, local media reported on Monday.

A Moscow district court sent all the four suspects, identified as citizens of Tajikistan, to pre-trial detention. They will remain in custody until May 22, Russia's TASS news agency reported.

On Friday night, gunmen fired indiscriminately at concertgoers at Crocus City Hall in suburban Moscow, leaving at least 137 people dead. A large blaze later led to the collapse of the building's roof.

Russian President Vladimir Putin said in a televised address that preliminary investigation showed the Ukrainian side was preparing a "window" for the perpetrators to cross the border. He promised to identify and punish all those behind the attack and declared March 24 a day of national mourning.