Russia observed a day of mourning by lowering flags to half-mast following the deadly attack at a concert outside Moscow on Friday night. Four men have been charged with the attack, which is considered the deadliest inside Russia in two decades, news agency Reuters reported. President Vladimir Putin declared Sunday a national day of mourning, vowing to punish all those responsible for the attack. The incident resulted in the deaths of 137 people, including three children, and left 182 injured.


More than 100 people are still hospitalized, with some in critical condition. Video footage depicted a solemn Putin lighting a candle at a church at his residence outside Moscow on Sunday evening to pay tribute to those who lost their lives.


Earlier on Sunday, individuals laid flowers at Crocus City Hall, the 6,200-seat concert hall outside Moscow where four armed men intruded just before the Soviet-era rock group Picnic was scheduled to perform its hit "Afraid of Nothing." The gunmen unleashed short bursts of gunfire at civilians, causing panic and screams among the crowd.


Islamic State has claimed responsibility for the attack, although Putin has not publicly linked the Islamist militant group to the perpetrators. Instead, he stated that the attackers were attempting to flee to Ukraine. Putin mentioned that some individuals on "the Ukrainian side" were willing to help the gunmen cross the border.


Ukraine has refuted any involvement in the attack.


On Sunday, Moscow's Basmanny district court charged four suspects with acts of terrorism related to the attack. The suspects were identified as Dalerdzhon Mirzoyev, Saidakrami Rachabalizoda, Shamsidin Fariduni, and Muhammadsobir Fayzov, according to the official Telegram channel of Moscow courts, Reuters reported.


The men, reportedly all citizens of Tajikistan residing in Russia, were remanded in pre-trial custody until May 22, according to Russian media reports. Three of the four suspects had pleaded guilty to all charges.


Following the circulation of unverified and brutal interrogation videos on social media, images from the courtroom published by Russian media depicted disturbing scenes: one suspect was brought in on a wheelchair, appearing to be missing an eye; another had a bandage where his right ear should be; a third had a black eye with a ripped plastic bag around his neck; and the fourth suspect, with a swollen face, seemed disoriented and struggled to keep his eyes open.