Russian authorities have detained 11 people, state media reported on Saturday, after gunmen stormed a concert hall in Moscow in a grisly attack that left at least 115 people dead.

Russia’s Investigative Committee said four of those detained were directly involved in the attack that left the sprawling shopping mall and music venue smoldering with a collapsed roof.

Russian agencies appeared to suggest the attack was linked to Ukraine even though the Islamic State group claimed responsibility in a statement.

A US intelligence official told The Associated Press that US agencies had confirmed that that group was responsible for the attack.

The four suspects were stopped in the Bryansk region of western Russia, “not far from the border with Ukraine,” Russia’s Investigative Committee said.

They planned to cross the border into Ukraine and “had contacts” there, state news agency Tass said, citing Russia’s FSB.

The head of the FSB briefed President Vladimir Putin on the arrests on Saturday, according to Tass.

At least three children were among those killed, Russian authorities said on Saturday, with hundreds more injured.

Images shared by Russian state media on 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 6000 people in Krasnogorsk, on Moscow’s western edge.

Videos posted online showed gunmen in the venue shooting civilians at point-blank range.

The roof of the theatre, where crowds had gathered on Friday for a performance by the Russian rock band Picnic, collapsed in the early hours of Saturday morning as firefighters spent hours battling a fire which erupted during the attack.

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.

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

The official said US intelligence agencies had gathered information in recent weeks that the IS 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 authorised 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 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 destabilise 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.

State media outlet TASS reported 145 people have been hospitalised. Sixty people are in a “serious condition.”

According to the Kremlin, Russian President Vladimir Putin was informed about the attack and is being kept updated on measures on the ground.

Putin on Saturday wished those were injured in the attack a speedy recovery, the state-run RIA-Novosti news agency. He also “conveyed his gratitude to the doctors,” RIA added.

Following the attack, Moscow City Duma Chairman Alexey Shaposhnikov called on Moscow residents to donate blood to help treat victims, saying that “this is a matter of life and death for dozens of people.”

Shaposhnikov listed several blood centre facilities in the Moscow area that will accept donors throughout the weekend.

Around 100 people were evacuated from the building by firefighters, TASS reported. Rescuers are still working to get people off the roof, according to the Main Directorate of the Ministry of Emergency Situations of the Russian Federation.

Moscow Mayor Sergei Sobyanin called the attack a “terrible tragedy.”

US had warned of potential attack

Earlier this month, the US embassy in Russia said it was “monitoring reports that extremists have imminent plans to target large gatherings in Moscow,” including concerts. The embassy warned US citizens to avoid large gatherings.

On Friday, following what it said were “reports of an ongoing terrorist incident at Crocus City Hall,” it advised US citizens not to travel to Russia.

US National Security Council spokesperson Adrienne Watson said the US government had information about a planned terrorist attack in Moscow – potentially targeting large gatherings, to include concerts – and that this is what prompted the State Department to issue the public advisory.

“The US government also shared this information with Russian authorities in accordance with its longstanding ‘duty to warn’ policy,” Watson said.

However, in a speech on Tuesday, Putin had blasted the American warnings as “provocative,” saying “these actions resemble outright blackmail and the intention to intimidate and destabilise our society”.

Ukraine, which has been embroiled in a war with Russia for more than two years, denied having any involvement in the attack.

“Ukraine has never resorted to the use of terrorist methods,” Ukrainian presidential adviser Mykhailo Podolyak wrote, in part, in a post on X. He said he believed Russia would use the attack to justify the ongoing conflict and scale up operations as part of “military propaganda” in Ukraine.

United Nations Secretary-General Antonio Guterres condemned “in the strongest possible terms today’s terrorist attack” according to a statement released by his deputy spokesperson, Farhan Haq.