In the aftermath of this weekend's mass shooting at a concert venue in Moscow's west, the country held a day of mourning on Sunday, March 24, with thousands gathering at the scene to place flowers at a makeshift memorial to remember the 137 people killed in the attack.

Russian authorities said that they arrested the four gunmen suspected of carrying out the attack near the border on Saturday, March 23, further alleging that they were from Tajikistan and had been in Russia on either temporary or expired visas.

State media named the suspects Dalerdzhon Mirzoyev, Saidakrami Rachabalizoda, Shamsidin Fariduni, and Muhammadsober Faizov.

The Islamic State earlier claimed responsibility for the assault in a short statement by the ISIS-affiliated Amaq news agency on Telegram on Friday, March 22.

Putin, Russia Gather to Grieve as Concert Hall Attack Marks Deadliest Day in Almost 20 Years

(Photo: MIKHAIL METZEL/POOL/AFP via Getty Images)

Russian President Vladimir Putin also turned out to pay his respects to the victims, lighting a candle in a church at his Novo-Ogaryovo residence outside Moscow.

"The whole country, our whole people, mourns with you," he told the nation in an address on Saturday.

The last time Russia experienced a mass shooting incident was during the Beslan school siege in 2004, CNN reported.

Read Also: Moscow Concert Massacre: Here's What We Know So Far

Members of the clergy also paid their respects and offered prayers for the sake of the victims, while Russian flags were flown at half-mast to indicate that the country was in mourning.

Prior to identifying the suspects, Putin claimed that the shooters were trying to escape to Ukraine, an allegation officials in Kyiv flatly denied.

"Ukraine was of course not involved in this terror attack," Ukrainian military intelligence service spokesperson Andriy Yusov said. "Ukraine is defending its sovereignty from Russian invaders, liberating its own territory and is fighting with the occupiers' army and military targets, not civilians."

Meanwhile, the United States stressed that they provided the Kremlin with an advanced warning about the attack, which Putin regarded as "blackmail" days prior to the attack.

Out of the 137 bodies recovered at the scene, only 62 were identified so far.

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