Eleven people have been detained after gunmen stormed a concert hall in Moscow and opened fire on the crowd, the head of Russia's Federal Security Service told President Vladimir Putin on Saturday, according to Russian state news agency Tass.
At least 93 people were killed in the attack, including three children, Russian authorities said Saturday.
Images shared by Russian state media Saturday showed a fleet of emergency vehicles still gathered outside the ruins of Crocus City Hall, a shopping mall and music venue with a capacity of more than 6,000 people in Krasnogorsk, on Moscow's western edge.
Friday's attack came just days after President Vladimir Putin cemented his grip on power in a highly orchestrated electoral landslide. The attack was the deadliest in Russia in years and came as the country's fight in Ukraine dragged into a third year.
Videos posted online showed gunmen in the venue shooting civilians at point-blank range. The roof of the theatre, where crowds had gathered Friday for a performance by the Russian rock band Picnic, collapsed in the early hours of Saturday morning as firefighters spent hours fighting a fire which erupted during the attack.
Four of those detained were directly involved in the attack, Tass said.
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The Islamic State group claimed responsibility for the attack in a statement posted on affiliated social media channels, although neither the Kremlin nor Russian security services have officially assigned blame for the attack.
In a statement posted by its Aamaq news agency, the Islamic State's affiliate in Afghanistan said it had attacked a large gathering of "Christians" in Krasnogorsk. It was not immediately possible to verify the authenticity of the claim.
However, a U.S. intelligence official told The Associated Press that U.S. intelligence agencies had confirmed that IS was responsible for the attack.
The official said U.S. intelligence agencies had gathered information in recent weeks that the IS branch was planning an attack in Moscow, and that U.S. officials had privately shared the intelligence earlier this month with Russian officials.
The official was briefed on the matter but was not authorized to publicly discuss the intelligence information and spoke to the AP on condition of anonymity.
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Messages of outrage, shock and support for those affected have since streamed in from around the world.
On Friday, the U.N. Security Council condemned "the heinous and cowardly terrorist attack" and underlined the need for the perpetrators to be held accountable. U.N. Secretary-General Antonio Guterres also condemned the terrorist attack "in the strongest possible terms," his spokesman said.
Meanwhile, in Moscow itself, hundreds of people stood in line Saturday morning to donate blood and plasma, Russia's health ministry said.
Putin, who extended his grip on Russia for another six years in this week's presidential vote after a sweeping crackdown on dissent, had publicly denounced the Western warnings of a potential terrorist attack as an attempt to intimidate Russians. "All that resembles open blackmail and an attempt to frighten and destabilize our society," he said earlier this week.
In October 2015, a bomb planted by the Islamic State downed a Russian passenger plane over Sinai, killing all 224 people on board, most of them Russian vacation-goers returning from Egypt. The group, which operates mainly in Syria and Iraq but also in Afghanistan and Africa, also has claimed several attacks in Russia's volatile Caucasus and other regions in the past years. It recruited fighters from Russia and other parts of the former Soviet Union.