MOSCOW (AP) — Russian authorities arrested the four men suspected of carrying out the attack on a suburban Moscow concert hall that killed at least 133 people, President Vladimir Putin said Saturday during an address to the nation. He claimed they were captured while fleeing to Ukraine.

Kyiv strongly denied any involvement in Friday’s attack on the Crocus City Hall music venue in Krasnogorsk, and the Islamic State’s Afghanistan affiliate claimed responsibility.

Putin didn’t mention IS in his speech, and Kyiv accused him and other Russian politicians of falsely linking Ukraine to the assault in order to stoke fervor in Russia’s war in Ukraine, which recently entered its third year.

A U.S. intelligence official told The Associated Press that U.S. agencies had confirmed that IS was responsible for the assault and that they had previously warned Moscow an attack could be imminent.

Putin said authorities have detained a total of 11 people in the attack, which also injured more than 100 concertgoers and left the venue a smoldering ruin. He called it “a bloody, barbaric terrorist act” and said Russian authorities captured the four suspected gunmen as they were trying to escape to Ukraine through a “window” prepared for them on the Ukrainian side of the border.

Russian media broadcast videos that apparently showed the detention and interrogation of the suspects, including one who told the cameras he was approached by an unidentified assistant to an Islamic preacher via a messaging app channel and paid to take part in the raid.

Russian news reports identified the gunmen as citizens of Tajikistan, a former Soviet country in Central Asia that is predominantly Muslim and borders Afghanistan. Up to 1.5 million Tajiks have worked in Russia and many received Russian citizenship.

Putin also said that additional security measures have been imposed throughout Russia, and he declared Sunday a day of mourning.

Some commentators on Russian social media questioned how authorities, who have relentlessly suppressed any opposition activities and muzzled independent media, failed to prevent the attack despite the U.S. warnings.

The attack came two weeks after the U.S. embassy in Moscow issued a notice urging Americans to avoid crowded places in view of “imminent” plans by extremists to target large Moscow gatherings, including concerts. The warning was repeated by several other Western embassies.

Investigators on Saturday were combing through the charred wreckage of the hall for more victims, and authorities said the death toll could still rise. Hundreds of people stood in line in Moscow early Saturday to donate blood and plasma, Russia’s health ministry said.

“We will identify and bring to justice all those stood behind the terrorists, all those who staged this atrocity, this assault against Russia and our people,” Putin said. “Russia has repeatedly faced hard, sometimes unbearable, trials, but it always came out even stronger.”

His claim that the attackers tried to flee to Ukraine followed comments by Russian lawmakers who pointed the finger at Ukraine immediately after the attack. But Mykhailo Podolyak, an adviser to Ukrainian President Volodymyr Zelenskyy, denied any involvement.

“Ukraine has never resorted to the use of terrorist methods,” he posted on X. “Everything in this war will be decided only on the battlefield.”

Ukraine’s foreign ministry accused Moscow of using the attack to try to stoke fervor for its war efforts.

“We consider such accusations to be a planned provocation by the Kremlin to further fuel anti-Ukrainian hysteria in Russian society, create conditions for increased mobilization of Russian citizens to participate in the criminal aggression against our country and discredit Ukraine in the eyes of the international community,” the ministry said in a statement.

Images shared by Russian state media Saturday showed emergency vehicles still gathered outside the ruins of Crocus City Hall, which could hold more than 6,000 people and has hosted many big events, including the 2013 Miss Universe beauty pageant that featured Donald Trump and others.

On Friday, crowds had gathered for a concert by the Russian rock band Picnic.

Videos posted online showed gunmen in the venue shooting civilians at point-blank range. Russian news reports cited authorities and witnesses as saying the attackers threw explosive devices that started the fire, which eventually consumed the building and caused its roof to collapse.

Dave Primov, who survived the attack, told the AP that the gunmen were “shooting directly into the crowd of people who were in the front rows.” He described the chaos in the hall as concertgoers rushed to leave the building: “People began to panic, started to run and collided with each other. Some fell down and others trampled on them.”

After he and others crawled out of the hall into nearby utility rooms, he said he heard pops from small explosives and smelled burning as the attackers set the building ablaze. By the time they got out of the massive building 25 minutes later, it was engulfed in flames.

“Had it been just a little longer, we could simply get stuck there in the fire,” Primov said.

Messages of outrage, shock and support for the victims and their families have 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.

IS, which lost much of its ground after Russia’s military action in Syria, has long targeted Russia. In a statement posted by the group’s Aamaq news agency, IS’s Afghanistan affiliate said it had attacked a large gathering of “Christians” in Krasnogorsk.

In October 2015, a bomb planted by IS 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.

On March 7, just hours before the U.S. embassy warned about imminent attacks, Russia’s top security agency said it had thwarted an attack on a synagogue in Moscow by an IS cell, killing several of its members in the Kaluga region near the Russian capital. A few days before that, Russian authorities said six alleged IS members were killed in a shootout in Ingushetia, in Russia’s Caucasus region.

A U.S. intelligence official told the AP that American 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.

Another U.S. official said the IS branch in Afghanistan had long targeted Russia and reiterated that no Ukrainians were involved in the attack.

Both officials were briefed on the matter but weren’t authorized to publicly discuss the intelligence information and spoke to the AP on condition of anonymity.

Just three days before the attack, Putin 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 at a meeting with top security officials.

Associated Press writers Michael Balsamo in Washington and Colleen Long in Wilmington, Delaware, contributed to this report.