Behind Moscow massacre: What we know about the attack

133 people were killed in an attack on a Moscow concert hall on Friday, the worst terrorist event in Russia for years. Islamic State (ISIS) has claimed responsibility, but Russian officials said the claim has not been verified. 

Four gunmen who attacked the concert were arrested on Saturday. Russian President Vladimir Putin said they were heading towards Ukraine and hoped to cross the border. Ukraine has denied any involvement in the attack.

Here is what is known so far:

Why were there so many casualties?

The deadliest attack in Russia since the 2004 Beslan school siege happened just before the rock group "Picnic" was to perform to a full house at the 6,200-seat Crocus City Hall just west of Moscow. The Crocus City complex is home to both the popular music hall and a shopping center.

Analysts pointed out that the attack was apparently well prepared, noting that the random shooting took place when crowds were at a peak on Friday night. The attack took place just after the Russian presidential election ended, and security in Moscow had been relaxed from the previous high level of alert.

Verified videos broadcast by Russian media and published on messaging app channels showed gunmen spraying civilians with bullets, as some victims lay motionless in pools of blood. One video showed the building on fire, with a huge cloud of smoke rising through the night sky. The street was lit up by the blinking blue lights of dozens of firetrucks, ambulances and other emergency vehicles, as fire helicopters buzzed overhead to dump water on a blaze that took hours to contain.

Preliminary results from an investigation by Russia's Investigative Committee show automatic weapons and flammable liquids were used in the attack. Russian news reports suggested more victims could have been trapped by the blaze that erupted after the assailants threw explosives.

Guards at the concert hall didn't have guns, and some may have been killed at the start of the attack, Russian media reported. Some Russian news outlets suggested that security measures such as cordons, patrols and safety checks at the entrance were not fully implemented before the attack.

Who was behind the attack? 

Russia's Federal Security Service on Saturday announced 11 people had been arrested, including four "directly involved" in the attack. Their identities have not been announced. Russia's Interior Ministry said they were foreign nationals.

Putin said that the four men were heading towards Ukraine when they were detained, and that they hoped to cross the border. "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," Putin stated.

TV editor Margarita Simonyan published a video on social media showing the interrogation of two suspects. One of the suspects, a young, bearded man, said he had flown from Turkey on March 4 and had received instructions from unknown people via Telegram to carry out the attack in exchange for money. Another man was shown being questioned via an interpreter while sitting on a bench with bound hands and feet.

In a statement posted by its Amaq news agency, the ISIS group said it was behind the attack. On Saturday, it released a photograph of what it said were the four attackers – all masked. It was not possible to verify the authenticity of the claim and Russian authorities have not commented. 

But U.S. intelligence officials have said the U.S. gathered information in recent weeks that an ISIS branch based in Afghanistan was planning an attack in Moscow, and that U.S. officials privately shared the intelligence with Russian officials. Russian officials said the U.S. intelligence lacked specific detail. 

The U.S. had singled out ISIS-Khorasan or ISIS-K, an offshoot of ISIS which seeks to establish a Muslim caliphate across Afghanistan, Pakistan, Turkmenistan, Tajikistan, Uzbekistan and Iran. U.S. counter-terrorism analysts said that the terrorist group has been fixated on Russia for the past two years as Moscow was involved in military campaigns against ISIS in Afghanistan, Chechnya, Syria and Iraq.

How did Putin respond?

Putin expressed condolences and promised assistance to the families of the victims and the injured. He declared March 24 a national day of mourning.

He cast the enemy as "international terrorism" and said he was ready to work with any state that wanted to defeat it.

"All the perpetrators, organizers and those who ordered this crime will be justly and inevitably punished. Whoever they are, whoever is guiding them," Putin said. "We will identify and punish everyone who stands behind the terrorists, who prepared this atrocity, this strike against Russia, against our people."

The Kremlin said Putin had held conversations with the leaders of Belarus, Uzbekistan and Kazakhstan in which all sides affirmed their willingness to work together to fight terrorism.

A senior Russian lawmaker, Andrei Kartapolov, said that if Ukraine was involved, then Russia must deliver a "worthy, clear and concrete" reply on the battlefield.

Russia has tightened security at airports, transport hubs and across the capital and big public events were cancelled across the country.

(With input from agencies)