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 three children were among the 60 people killed, 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 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 theater, 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.

The ISIS 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, ISIS'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 US intelligence official told The Associated Press that US intelligence agencies had confirmed that ISIS was responsible for the attack.

The official said US intelligence agencies had gathered information in recent weeks that the ISIS branch was planning an attack in Moscow, and that US 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.

Messages of outrage, shock and support for those affected have since streamed in from around the world.

On Friday, the UN Security Council condemned “the heinous and cowardly terrorist attack” and underlined the need for the perpetrators to be held accountable. UN 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, 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.

ISIS, 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.