MOSCOW (AP) — Russian authorities have arrested 11 people, state media reported Saturday, after gunmen stormed a concert hall in Moscow in a gruesome attack that left at least 115 people dead.

According to the Russian Investigative Committee, four of those arrested were directly involved in the attack that left the sprawling shopping center and music hall smoldering under a collapsed roof.

Russian authorities appeared to suspect the attack was linked to Ukraine, although the Islamic State group claimed responsibility in a statement. A U.S. intelligence official told the Associated Press that U.S. authorities had confirmed that group was responsible for the attack.

The four suspects were stopped in the Bryansk region in western Russia, “not far from the border with Ukraine,” the Russian Investigative Committee said. They planned to cross the border into Ukraine and had “contacts” there, the state news agency Tass said, citing the Russian FSB. According to Tass, the head of the FSB informed President Vladimir Putin about the arrests on Saturday.

The attack came just days after Putin consolidated his grip on power with an elaborately staged electoral landslide. The attack was Russia’s deadliest in years and came as the country’s fight in Ukraine dragged into its third year.

Shortly after the attack, some Russian lawmakers pointed the finger at Ukraine. Mykhailo Podolyak, an adviser to Ukrainian President Volodymyr Zelensky, denied any involvement.

“Ukraine has never resorted to terrorist methods,” he posted on X, formerly Twitter. “Everything in this war is decided only on the battlefield.”

Images shared by Russian state media on Saturday showed a fleet of emergency vehicles still gathered outside the ruins of the Crocus City Hall, which held more than 6,000 people in Krasnogorsk on Moscow’s western edge.

Videos posted online showed gunmen shooting civilians at close range at the venue. The roof of the theater where crowds had gathered on Friday for a performance by Russian rock band Picnic collapsed in the early hours of Saturday as firefighters battled a fire that broke out during the attack for hours.

In a statement published by the Aamaq news agency, the Islamic State in Afghanistan affiliate said it attacked a large gathering of “Christians” in Krasnogorsk. It was not immediately possible to verify the authenticity of the claim.

A U.S. intelligence official told the AP that U.S. intelligence agencies had been gathering information in recent weeks that the IS affiliate was planning an attack in Moscow, and that U.S. officials privately shared the information with Russian officials earlier this month.

The official was briefed on the matter but was not authorized to discuss the intelligence publicly and spoke to the AP on condition of anonymity.

Since then, messages of outrage, shock and support for those affected have poured in from around the world.

On Friday, the UN Security Council condemned “the despicable and cowardly terrorist attack” and stressed the need to hold the perpetrators accountable. UN Secretary-General Antonio Guterres also condemned the terrorist attack “in the strongest possible terms,” his spokesman said.

Meanwhile, hundreds of people lined up to donate blood and plasma in Moscow on Saturday morning, the Russian Health Ministry said.

Putin, who extended his influence over Russia for another six years in this week’s presidential election after a sweeping crackdown on dissent, had publicly denounced Western warnings of a possible terrorist attack as an attempt to intimidate Russians. “This all resembles outright blackmail and an attempt to intimidate and destabilize our society,” he said earlier this week.

In October 2015, a bomb planted by the Islamic State crashed a Russian passenger plane over Sinai, killing all 224 people on board, most of them Russian vacationers returning from Egypt. The group, which operates primarily in Syria and Iraq but also in Afghanistan and Africa, has also claimed several attacks in the volatile Russian Caucasus and other regions in recent years. It recruited fighters from Russia and other parts of the former Soviet Union.

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Associated Press writer Michael Balsamo in Washington contributed to this report.