Vladimir Putin has linked the deadliest attack in Russia for years to Ukraine – despite a branch of Islamic State claiming responsibility.
The Russian president may have ignored Western intelligence that came two weeks ago warning the terrorist group was planning an attack.
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Here Sky News looks at what we know so far.
When did the attack unfold?
On Friday evening, the rock band Piknik had just come on stage at Crocus City Hall when a group of gunmen stormed the venue and started firing indiscriminately.
The four men arrived at the venue, which has a capacity of 6,200 people and sits 15 miles west of Moscow, in a minivan at around 7.40pm.
Gunmen are seen inside the venue
Kalashnikov on the floor of the venue. Pic: Russian Investigative Committee
They approached the metal detectors at the entrance carrying Kalashnikovs and began shooting people at point-blank range.
Video footage from inside the concert hall shows people screaming and running towards the exit as the gunmen fired in short bursts.
They are then believed to have poured liquid around the hall, igniting it before fleeing.
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Survivor describes ordeal
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Who are IS-K and why would they target Russia?
Ukraine worried Putin will exploit Moscow bloodshed
Margarita Bunova, who was in the crowd and managed to escape, said: “I heard a blast. I thought it was a firecracker. But these crackles, they weren’t stopping.
“There was screaming, panic. When they started shooting, the people themselves were already downstairs. My husband grabbed me and we ran through the upper floor through the fire safety.”
As emergency responders scrambled to the scene, the hall went up in flames, with part of the roof collapsing.
They were situated around two miles away and took roughly half an hour to arrive, according to Sky’s security and defence analyst Professor Michael Clarke.
Emergency vehicles outside Crocus City Hall. Pic: Reuters
How many have died?
According to Russia’s investigative committee, 133 people have been confirmed dead so far.
At least 145 people were injured, with more than 100 still in hospital.
Men cover themselves with blankets near the burning Crocus City Hall. Pic: Reuters
Who were the attackers?
The Russian Security Service (FSB) has said it arrested 11 people, including the four men believed to have carried out the attack.
They fled the scene in a white car and were detained in Bryansk, a region around 200 miles southwest of Moscow.
Footage being circulated by Russian media on Telegram shows some men being apprehended at the side of a road. One is heard saying: “I shot people,” as his hands are tied behind his back.
One of the suspects gave the name Shamsutdin Fariddun and said he was born on 17 September 1998.
Another, with facial injuries, spoke in Tajik via a translator and gave his name as Rajab Alizadeh.
A massive blaze is seen at the Crocus City Hall. Pic: AP
Who has claimed responsibility?
The men are reportedly members of Islamic State Khorasan (IS-K), which issued a statement claiming responsibility for the attack four hours after it happened.
It said: “The attack comes within the context of a raging war between the Islamic State and countries fighting Islam.”
IS-K, an offshoot of ISIS, has vowed to form a caliphate across Afghanistan, Pakistan, Turkmenistan, Tajikistan, Uzbekistan, and Iran.
It is largely made up of former Pakistani Taliban fighters and recruits members across central Asia and Russia.
US officials have said they have intelligence that confirms IS-K were behind the atrocity.
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Why would IS-K attack Russia?
What has Vladimir Putin said?
Despite the IS-K admission, Mr Putin has suggested the suspects had links to Ukraine and were trying to flee there when they were arrested.
He said in a video address on Saturday: “They tried to hide and moved towards Ukraine, where according to preliminary data, a window was prepared for them from the Ukrainian side to cross the state border.”
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Putin vows to punish perpatrators
He described the incident as “international terrorism”.
Kyiv has vehemently denied any involvement.
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Mr Putin vowed to punish those responsible, adding: “All the perpetrators, organisers and those who ordered this crime will be justly and inevitably punished. Whoever they are, whoever is guiding them.
“We will identify and punish everyone who stands behind the terrorists, who prepared this atrocity, this strike against Russia, against our people.”
Crocus City Hall on Saturday. Pic: Reuters
What has the West said?
The US has said it had passed the Kremlin intelligence that terrorists were planning an attack in Moscow two weeks before Friday’s incident.
It said IS was planning an atrocity on a conference or concert venue.
US National Security Council spokesman Adrienne Watson said in a statement: “Earlier this month, the US government had information about a planned terrorist attack in Moscow – potentially targeting large gatherings, to include concerts – which prompted the State Department to issue a public advisory to Americans in Russia.
“The US government also shared this information with Russian authorities in accordance with its longstanding ‘duty to warn’ policy.”