Late Friday, gunmen armed with automatic weapons attacked the Crocus City Hall — a massive shopping and entertainment venue on the outskirts of Moscow — and set the concert hall alight. The assault followed U.S. government warnings this month about a “planned terrorist attack” in the Russian capital.

Russia’s Investigative Committee said more bodies were found at the site Saturday, adding that the death toll was expected to rise, with 16 of the 107 hospitalized victims in grave condition and 44 in serious condition.

Putin called the attack a planned and an organized mass murder of innocent and defenseless people, and he promised swift retaliation.

“The criminals in cold blood, purposefully went to kill and shoot at point-blank range our citizens and our children, as the Nazis did who committed massacres in the occupied territories. They planned to stage a demonstrative execution, a bloody act of intimidation,” he said. “All perpetrators, organizers and sponsors of this crime will be fairly and unavoidably punished, whoever they are or whoever directs them.”

The Islamic State claimed responsibility for the Friday night attack, already one of the most deadly in modern Russian history, which left about 140,000 square feet of the venue in Krasnogorsk in flames, according to Russia’s emergency services. A U.S. official, speaking on the condition of anonymity to discuss sensitive information, told The Washington Post that the United States had “no reason to doubt” the claim from the Islamic State.

The Investigative Committee said that the preliminary causes of death were gunshot wounds and inhalation of fumes. The committee said investigations were continuing at the venue.

By Saturday afternoon, the Health Ministry had identified 41 of the victims, predominantly people between the ages of 30 and 60.

Baza, a Telegram channel with ties to Russian security services, reported that 28 bodies were found in one of the venue’s bathrooms, which included “many mothers” clasping their children, while another 14 were discovered in an emergency exit stairwell.

The attack — which indicated Russia’s continued vulnerability to terrorist attacks — came just days after Putin’s victory in a highly orchestrated election, which solidified his power as his war in Ukraine drags into its third year.

Putin’s speech was his first public statement after the attack. Before that, Russian Deputy Prime Minister Tatyana Golikova said the president “wished everyone recovery” and thanked the doctors, Tass reported.

Kremlin spokesman Dmitry Peskov said Saturday that Putin spoke to Belarusian President Alexander Lukashenko by phone and the two leaders “reiterated their readiness to cooperate in countering terrorism.” Lukashenko offered his condolences over the attack, Peskov added.

The U.S. government issued a public advisory to Americans in Russia on March 7 that described the risk of a “planned terrorist attack in Moscow — potentially targeting large gatherings, to include concerts,” National Security Council spokeswoman Adrienne Watson said in a statement Friday.

“The U.S. Government also shared this information with Russian authorities in accordance with its long-standing ‘duty to warn’ policy,” Watson said.

The warning was based, in part, on intelligence reporting about possible activity inside Russia from Islamic State-Khorasan, the Afghanistan and Pakistan arm of the Islamic State, two U.S. officials who also spoke on the condition of anonymity told The Post. Other Western embassies echoed the warning.

Mykhailo Podolyak, an adviser to Ukrainian President Volodymyr Zelensky, has denied any responsibility for Friday’s attack, writing on social media that Ukraine “certainly has nothing to do with the shooting/explosions” and that “everything in this war will be decided only on the battlefield.”

Ukraine’s military intelligence agency blamed Russia’s security services for the operation and said they would probably use the fallout to build support for their war in Ukraine and possibly to launch a second round of mobilization.

Accusations of responsibility have swirled in the wake of the attack, with several senior Russian officials, including former president Dmitry Medvedev, appearing to placing the blame on Kyiv and threatening harsh retaliation.

On Friday, Russian oligarch Konstantin Malofeev called for a nuclear strike.

“Let’s give the civilian population of Ukraine 48 hours to leave the cities and finally end this war with the victorious defeat of the enemy. Using all forces and means,” Malofeev wrote on Telegram.

The Federal Security Services, or FSB, public relations office said Saturday that the assailants were “intending to cross the border into Ukraine,” where they had contacts.

Andriy Yusov, a spokesman for Ukraine’s military intelligence, told The Post that warnings had circulated ahead of Friday’s attack, which he connected to Russia’s massive barrage of missile and drone strikes on Ukraine early Friday. The attacks on regions across Ukraine targeted key energy infrastructure, knocked out power in several places and killed at least five people.

“Today’s attack on the Ukrainian energy system, Ukrainian cities and a terrorist attack at Crocus City Hall are components of one plan and one operation,” Yusov said.

Videos from Friday, verified by The Post, show four men in camouflage entering a large hall before the start of a sold-out concert and opening fire, as well as shooters firing rounds inside a concert hall, thick with smoke. Other footage shows numerous bodies slumped on the floor.

Speaking at a news conference Friday, White House national security spokesman John Kirby called the images “just horrible and just hard to watch.”

U.N. Secretary General António Guterres condemned “in the strongest possible terms today’s terrorist attack” and expressed “deep condolences,” his spokesperson said in a statement Friday.

Moscow Mayor Sergei Sobyanin on Telegram called Friday’s attack a “terrible tragedy” and wrote that all sports, cultural and public events in Moscow would be canceled over the weekend.

Opposition figure Yulia Navalnaya — whose husband, staunch Putin critic Alexei Navalny, died in a Russian prison colony last month — called the attack a “nightmare.”

“Everyone involved in this crime must be found and held accountable,” she said.

Makeshift memorials and billboards with a single candle and the words “we mourn” have appeared in cities and highways across Russia to mark Friday’s attack. People stood in long lines throughout the capital Saturday to donate blood. The Russian Foreign Ministry has ordered the lowering of flags on buildings of Russian embassies abroad.

Putin has called for a national day of mourning to be held on Sunday.

Siobhán O’Grady and Natalia Abbakumova contributed to this report.