MOSCOW (AP) — Russian authorities arrested 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 in an address to the nation. He claimed they were captured while fleeing to Ukraine.

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

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

U.S. intelligence officials confirmed the claim by the IS affiliate that it was responsible for the attack, a U.S. official told The Associated Press. U.S. intelligence agencies gathered information in recent weeks that the IS branch was planning an attack in Moscow, and U.S. officials privately shared the intelligence with Russian officials earlier this month, the U.S. official said. 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.

Putin said authorities detained a total of 11 people in the attack, which also injured more than 100 concertgoers and left the venue on Moscow’s western rim 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 and paid to take part in the raid.

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

Tajikistan's foreign ministry, which denied initial Russian media reports that mentioned several other Tajiks allegedly involved in the raid, did not immediately respond to a request for comment about Saturday's arrests.

Many Russian hard-liners called for a crackdown on Tajik migrants, but Putin appeared to reject the idea, saying “no force will be able to sow the poisonous seeds of discord, panic or disunity in our multi-ethnic society.”

He declared Sunday a day of mourning and said additional security measures were imposed throughout Russia.

The attack, the deadliest in Russia in years, is a major embarrassment for the Russian leader and happened just days after he cemented his grip on the country for another six years in a vote that followed the harshest crackdown on dissent since the Soviet times.

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 assault 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. Several other Western embassies repeated the warning. Earlier this week, Putin denounced the warning as an attempt to intimidate Russians.

Investigators on Saturday combed 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 to donate blood and plasma, Russia’s health ministry said.

Putin's 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 build support 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 showed emergency vehicles still gathered outside the ruins of Crocus City Hall, which could hold more than 6,000 people and hosted many big events, including the 2013 Miss Universe beauty pageant that featured Donald Trump.

On Friday, crowds were at the venue 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" in the front rows. He described the chaos in the hall as concertgoers raced to escape: “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 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.

On Saturday, the group issued a new statement on Aamaq saying the attack was carried out by four men who used automatic rifles, a pistol, knives and firebombs. It said the assailants fired at the crowd and used knives to kill some concertgoers, casting the raid as part of IS's ongoing war with countries that it says are fighting Islam.

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.

The group’s Afghanistan affiliate is known variously as ISIS-K or IS-K, taking its name from Khorasan Province, a region that covered much of Afghanistan, Iran and Central Asia in the Middle Ages.

The affiliate has thousands of fighters who have repeatedly carried out attacks in Afghanistan since the country was seized in 2021 by the Taliban, a group with which they are at bitter odds.

ISIS-K was behind the August 2021 suicide bombing at Kabul airport that left 13 American troops and about 170 Afghans dead during the chaotic U.S. withdrawal. They also claimed responsibility for a bomb attack in Kerman, Iran, in January that killed 95 people at a memorial procession.

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 and killed 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.

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