MOSCOW — The death toll in the Moscow concert hall attack has risen to 133, Russia's top state investigative agency said Saturday.

The update from the Investigative Committee comes as authorities are combing the charrred wreckage of the Crocus City Hall on Moscow's western edge for more victims. Officials previously put the death toll from Friday's raid at 115.

The attack also left many wounded.

The Islamic State group's Afghanistan branch claimed responsibility for the attack in a statement posted on affiliated channels on social media. A U.S. intelligence official told The Associated Press that U.S. agencies had confirmed that the group was responsible for the attack.

In an address to the nation on Saturday, Russian President Vladimir Putin said authorities had detained 11 people, including four who took part in the attack. He also suggested they had been trying to cross the border into Ukraine which, he said, tried to create a "window" to help them escape.

Ukraine has strongly denied any involvement and accused Moscow of using the attack to try to stoke fervor in its war effort.

In an address to the nation, Putin called it "a bloody, barbaric terrorist act" and said all four people who were directly involved had been taken into custody. He suggested they had been trying to cross the border into Ukraine which, he said, tried to create a "window" to help them escape.

Ukraine has strongly denied any involvement in the attack. Putin said Saturday that additional security measures have been imposed throughout the country and declared March 24 a day of national mourning.

The Islamic State group's Afghanistan branch claimed responsibility for Friday's attack in a statement posted on affiliated channels on social media. A U.S. intelligence official told The Associated Press that U.S. agencies had confirmed that the group was responsible for the attack.

The attack, which was the deadliest in Russia in years, came just days after Putin cemented his grip on power in a highly orchestrated electoral landslide and as the country's war in Ukraine dragged into a third year.

Some Russian lawmakers 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, formerly Twitter. "Everything in this war will be decided only on the battlefield."

Ukraine's foreign ministry also denied that the country had any involvement and 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," a ministry said in a statement.

Images shared by Russian state media Saturday showed a fleet of emergency vehicles still gathered outside the ruins of Crocus City Hall, which had a maximum capacity of more than 6,000 people.

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. The roof of the theater, where crowds had gathered for a performance by the Russian rock band Picnic, collapsed early Saturday as firefighters spent hours fighting the blaze.

In a statement posted by its Aamaq news agency, the I.S.'s Afghanistan affiliate said it had 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 American intelligence agencies had gathered information in recent weeks that the I.S. branch was planning an attack in Moscow, and that U.S. 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 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.

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Camouflage-clad gunmen opened fire at concertgoers with automatic weapons on Friday, killing at least 60 people and injuring 145 more in an attack claimed by Islamic State militants.

Meanwhile hundreds of people stood in line on Saturday in Moscow 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 destabilize our society," he said earlier this week.

In October 2015, a bomb planted by I.S. 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.

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