Moscow attack: Day of mourning after 137 killed at Crocus City Hall concert

Russia is observing a day of mourning after at least 137 people were killed in an attack on Friday evening at a packed concert venue in Moscow.

Flags are flying at half mast, many events have been cancelled and TV channels have updated their schedules.

More than 140 people were also injured when gunmen entered the Crocus City Hall, firing indiscriminately before setting it on fire.

The Islamic State (IS) group says it was behind the attack.

On Saturday Amaq, the IS media outlet, posted an image of the four masked men they claimed were involved in the assault. Russia has not commented on the IS claim.

The group later released highly graphic footage from the attack. The video, which has been verified by the BBC as genuine, shows one of the gunmen opening fire on several people. The BBC will not be broadcasting this video.

Russian President Vladimir Putin has said all four gunmen who carried out the attack have been arrested.

On Sunday, Russia's Investigative Committee showed the detainees being taken from a van into a building, handcuffed and blindfolded.

In his televised address on Saturday, Mr Putin condemned the massacre - the deadliest in Russia for nearly 20 years - as a "barbaric terrorist act" and repeated earlier suggestions by Russian security services that the attackers had tried to escape to Ukraine.

Kyiv dismissed the claim that it was in some way involved in the attack as "absurd".

"To suggest the suspects were heading to Ukraine, would suggest they were stupid or suicidal," Andriy Yusov, a spokesperson for the Ukrainian military intelligence directorate, told the BBC.

Ukrainian President Volodymyr Zelensky accused Mr Putin of seeking to "blame" Ukraine for the attack. "This worthless Putin, instead of dealing with his citizens of Russia, addressing them, remained silent for a day - thinking how to bring this to Ukraine," he said in his nightly address.

BBC Verify has been able to match details of two of the alleged attackers who appear in the IS video and an IS-released still image to videos of the arrested suspects posted on pro-Kremlin Telegram channels.

The US National Security Council said it warned Russia of a potential IS attack on "large gatherings", including concerts, in Moscow earlier this month. The Kremlin at the time dismissed that as "propaganda" and an attempt by Washington to meddle in the recent Russian election.

On Saturday, the White House said it condemned the "heinous" attack and described IS as "a common terrorist enemy that must be defeated everywhere".

Reports of a serious attack inside the Crocus City Hall, in Moscow's Krasnogorsk suburb, began arriving at around 20:00 local time (17:00 GMT) on Friday.

Up to 6,200 people were gathered there for a Friday night rock concert by veteran band Picnic when the shooting started.

One video posted online showed several men striding across the concourse, where they opened fire at members of the public, before rearming and entering the hall.

Crowds of people were filmed screaming and running in panic as the men burst in. Others were seen taking cover behind their seats as the men fired into the auditorium.

Some of those who tried to escape from the gunmen were thought to have fled to the basement, and others to the roof.

"They were just walking and gunning down everyone methodically in silence. Sound was echoing and we could not understand what was where," concertgoer Anastasia Rodionova recalled.

Vitaly, another visitor, saw the attack unfolding from a balcony. "They threw some petrol bombs, everything started burning," he said.

Outside the hall, huge plumes of smoke filled the sky. A fire was later seen engulfing the hall's roof and facade. Tass state news agency reported that around a third of the building had been set alight.

Other reports spoke of explosions, the force of which shattered the glass on the top two floors of the structure.

A number of special forces units stormed the venue, while medical personnel, and dozens of ambulances were sent to the scene. Helicopters, circling overhead, tried to douse the fire.

The Russian Investigative Committee said that the attackers used flammable liquid to set fire to sections of the concert hall and that the deaths of the victims were caused by gunshot wounds and poisoning by combustion products.

On Saturday, a makeshift memorial was set up outside the concert hall where Muscovites lit candles and laid flowers. Others queued to donate blood for the victims of the massacre.

In Moscow and other towns and cities, electronic billboards displayed the image of a single burning candle and the word "Skorbim" - "we mourn."