Russian President Vladimir Putin on Saturday vowed to punish those behind a "barbaric terrorist attack" on a Moscow concert hall that killed more than 130, saying Russia had arrested four gunmen who were trying to flee to Ukraine.
Kyiv has strongly denied any connection, and Putin made no reference to claims of responsibility by the Islamic State group (IS) in his first public remarks on the attack.
At least 133 people were killed when camouflaged gunmen stormed the Crocus City Hall, in Moscow's northern suburb of Krasnogorsk, and then set fire to the building on Friday evening.
The jihadist group has claimed the attack, writing Saturday on a Telegram channel that it was "carried out by four IS fighters armed with machine guns, a pistol, knives and firebombs," as part of "the raging war" with "countries fighting Islam".
It is the deadliest attack in Russia for almost two decades and the deadliest in Europe to have been claimed by IS.
Russian officials expect the death toll to rise further, with more than 100 injured in hospital.
"Terrorists, murderers, non-humans ... have only one unenviable fate: retribution and oblivion," Putin said in a televised address to the nation on Saturday.
in what has been widely condemned as a sham electoral landslide.
Here's what we know so far.
What happened?
Gunmen burst into the and began shooting automatic weapons at civilians who were there to see Soviet-era rock group "Picnic" perform to a full house at the 6,200-seat theatre.
At least five gunmen dressed in camouflage took part in the attack, which was captured on video by panicked concertgoers.
A verified video shows people taking their seats in the hall and then rushing for the exits as repeated gunfire echoed above screams. Other videos showed men shooting at groups of people. Some victims lay motionless in pools of blood.
"Suddenly there were bangs behind us — shots. A burst of firing — I do not know what," one witness, who asked not to be identified by name, told Reuters.
"A stampede began. Everyone ran to the escalator," the witness said. "Everyone was screaming; everyone was running."
Who is behind the Crocus City Hall attack?
, which once sought control over swathes of Iraq and Syria, claimed responsibility for the attack.
The Afghan branch of IS, known as ISIS-K or IS-K, said its fighters attacked on the outskirts of Moscow, "killing and wounding hundreds and causing great destruction to the place before they withdrew to their bases safely".
The statement gave no further detail.
While Friday's attack in Russia was a dramatic escalation, experts say the group has opposed Russian President Vladimir Putin in recent years.
The attack on Crocus City Hall, about 20 km from the Kremlin, comes just two weeks after the US embassy in Russia warned that "extremists" had imminent plans for an attack in Moscow.
The United States has intelligence confirming the IS group's claim of responsibility for the shooting, a US official said on Saturday.
"ISIS-K has been fixated on Russia for the past two years, frequently criticising Putin in its propaganda," said Colin Clarke, with the Soufan Center. There is also a Caucasus branch of the IS group.
What has been the reaction abroad?
A White House spokesperson said the images of the attack were "horrible" and "hard to watch", adding that "our thoughts, obviously, are going to be with the victims of this terrible, terrible shooting attack".
Ukraine's presidency said Kyiv had "nothing to do" with the attack, while its military intelligence called the incident a Russian "provocation" and charged that Moscow special services were behind it.
The Freedom of Russia Legion, a pro-Ukrainian militia responsible for attacks on Russia's border regions, also denied any role.
Former Russian president Dmitry Medvedev vowed on Telegram that Ukraine's top officials "must be found and ruthlessly destroyed as terrorists" if they were linked to the attack.
The European Union, France, Spain and Italy joined several countries in condemning the attack, with Italian premier Giorgia Meloni denouncing "an odious act of terrorism".
Has this happened before?
In the early 2000s when Russia was at war with Chechen separatists, the country suffered a spate of terror attacks.
In October 2002, a major hostage situation unfolded at a theatre in Moscow, when militants took hundreds of people hostage. When Russian special forces stormed the building two days later, 129 hostages and 41 Chechen fighters died.
The death toll of the Crocus City Hall attack made it one of the worst attacks on Russia since the 2004 Beslan school siege, when Islamist militants took more than 1,000 people, including hundreds of children, hostage.
