The Russian authorities said on Saturday that they had arrested the four individuals suspected of setting a suburban Moscow concert on fire and killing at least 133 people, one of the worst terrorist attacks to jolt Russia in President Vladimir V. Putin’s nearly quarter century in power.
The Islamic State has taken responsibility for the brutal assault in three different messages issued since Friday. But Mr. Putin, in his first public remarks on the tragedy more than 19 hours after the attack, made no mention of the extremist group or the identities of the perpetrators, broadly blaming “international terrorism,” while Russian state media quickly began laying the groundwork to suggest that Ukraine and its Western backers were responsible.
The Russian leader did take a swipe at Ukraine, saying that the suspects were apprehended while traveling to the Russian border, where he alleged a crossing was being prepared for them from “the Ukrainian side.” Kyiv has denied any involvement in the attack.
Russian state news broadcasts largely ignored or cast doubt on the ISIS attribution, and commentators focused on trying to blame Ukraine. As of Saturday, the authorities had not disclosed the identities of the alleged gunmen.
But state news media did show what it described as footage of interrogations of at least two of the suspects, including one who spoke in Tajik through an interpreter and another who said he carried out the killings for money after being recruited over the messaging app Telegram. Russia’s Interior Ministry said the four suspects were all foreign citizens.
In his video address, Mr. Putin said the four main perpetrators had been apprehended, as well as seven other individuals.
“The main thing now is to prevent those who were behind this bloody massacre from committing new crimes,” he said.
The Russian leader designated Sunday as a national day of mourning and vowed retribution against those who organized the attack.
“All perpetrators, organizers and commissioners of this crime will receive a just and inevitable punishment,” Mr. Putin said. “No matter who they are, no matter who directed them, I repeat, we will identify and punish everyone who stood behind the terrorists.”
By Saturday, the vast concert venue had been reduced to a heap of burned rubble, dust and smoke, after a mammoth fire engulfed the premises in the hours after the attack and pulled down the roof.
As emergency services continued to comb the scene, survivors gave harrowing accounts of their escapes.
“The panic was terrifying,” said Olya Muravyova, 38, who had been standing in line with her husband to buy a beer before the performance by Piknik, a Russian rock band formed in the late 1970s that was about to play at the venue when the attack occurred.
“We were in such a good mood,” she said on Saturday, visiting the scene of the attack in the hopes of picking up her car. Suddenly, five minutes before the performance was set to start, she heard shots ring out.
“I thought maybe the band was making a dramatic entrance,” she said. But her husband told her to run, and then to hide.
The names of some of the victims have also begun to emerge from officials and in local news reports. Most of those identified so far appeared to have been in their 40s, and many had traveled from other parts of the country to attend the concert.
Alexander Baklemishev, 51, had long dreamed about seeing the band, his son told local media, and had traveled from his home city of Satka, some 1,000 miles east of Moscow, to see them perform.
His son, Maksim, told the Russian news outlet MSK1 that his father had sent a video of the concert hall before the attack and that was the last he heard from him.
“There was no last conversation,” his son said. “All that was left is the video, and nothing more.”
On Saturday night, the governor of the Moscow region announced that rescuers had ended the search for survivors at the suburban Moscow concert venue, according to TASS. The death toll remained at 133, but the search for bodies would continue, the governor said.
Across the country, Russians placed flowers at makeshift memorials. Many lined up in the capital to donate blood. Russian officials gave regular updates about the more than 100 people wounded in the attack, many of them in critical condition. The authorities warned that the death toll was likely to rise, and said three children were among the dead.
United States officials said the atrocity was the work of Islamic State-Khorosan, or ISIS-K, an offshoot of the group that has been active in Pakistan, Afghanistan and Iran.
“ISIS is a common terrorist enemy that must be defeated everywhere,” Karine Jean-Pierre, the White House press secretary, said on Saturday.
Secretary of State Antony J. Blinken said the United States condemns the attack in Moscow and “stands in solidarity with the people of Russia grieving the loss of life after this horrific event.”
The tragedy began on Friday evening, when men in fatigues armed with automatic weapons stormed Crocus City Hall, situated in the Moscow suburb of Krasnogorsk.
First, they began shooting people, many at point-blank range. Then, the attackers used a flammable liquid to set fire to the premises of the large concert hall, according to Russia’s Investigative Committee, which said many of the victims perished after inhaling the toxic fumes.
In interviews with Russian media, some of the concert attendees recalled running out of the venue and trying to escape through a utility area, only to find the doors locked.
The attack represented a significant security failure for the Kremlin, and came just days after Mr. Putin claimed victory in the presidential election.
For years, Mr. Putin has emphasized countering international terrorism as a top priority, but since invading Ukraine two years ago, he has pivoted to casting the West as the biggest foreign threat faced by Russians.
The lapse raised questions about whether Mr. Putin’s security services, which have been concentrating squarely on waging war against Ukraine, overlooked the threat posed by extremist Islamic groups. Russia has long been a target for Sunni extremists, because of its backing of Syria and Iran, and the country for years faced extremist attacks emanating from its own North Caucasus region.
At least 128 people died when Chechen extremists took a Moscow theater hostage in 2002 during a performance of the musical “Nord-Ost.” Two years later, Chechen militants besieged a school in Beslan, a national tragedy that killed more than 330 people, more than half of them children.
More recently, the Islamic State claimed responsibility for shooting down a Russian aircraft taking off from Egypt in 2015. An Al Qaeda-linked group claimed responsibility for an attack on the St. Petersburg metro in 2017.
In recent weeks, the Russian authorities had been warned about the possibility of a terrorist attack at a concert in Moscow.
On March 7, the American Embassy in Moscow issued a rare, specific public warning calling on people to avoid large gatherings, including concerts, owing to information that extremists had imminent plans to target such events in the Russian capital.
The public warning came after the United States collected intelligence suggesting that ISIS-K was planning an attack in Moscow, U.S. officials told The New York Times. Beyond the Embassy’s public warning, U.S. officials also privately told Russian officials about intelligence suggesting an impending attack, the officials said.
During a March 19 speech to the Federal Security Service, Mr. Putin dismissed the Western warnings as “outright blackmail” and attempts “to intimidate and destabilize our society.”
After Friday’s attack, Russian state propagandists tried to suggest that the advance warning provided by the United States meant that Washington had a hand in the attack. But Mr. Putin, beyond blaming unspecified individuals on the Ukrainian side for preparing a border crossing, stopped short of making any such accusations.
“We know what the threat of terrorism is,” Mr. Putin said. “We are counting here on cooperation with all countries that genuinely share our pain and are ready, in their deeds, to truly unite efforts in the fight against the common enemy of international terrorism.”
The guests on a political talk show on Russia’s flagship Channel One rushed to find ways to blame Ukraine on Saturday evening, suggesting without evidence that Kyiv had to be behind the attack, despite Islamic State’s claims of responsibility.
Leonid Reshetnikov, a former top Russian intelligence officer, accused Ukraine of turning to terrorism because its forces couldn’t win on the battlefield.
“So long as this kind of government, this kind of regime exists, this terror will continue,” Mr. Reshetnikov said on the show, noting that Moscow needed to “end” Ukraine as a government established on Russian land.
Responding to the Kremlin’s accusations, Ukraine’s president, Volodymyr Zelensky, called Mr. Putin a “nonentity” who sent hundreds of thousands of Russians to fight in Ukraine rather than protect their own country.
“They came to Ukraine and are burning our cities and they try to blame Ukraine,” Mr. Zelensky said in a video posted to Telegram on Saturday evening.
Crocus City Hall, the concert hall where the attack took place, opened in 2009 as one of the glitziest new venues in the Russian capital. It went on to host top international acts, including Eric Clapton, Sia and Lorde, as well as Donald J. Trump’s Miss Universe pageant in 2013.
Pictures published by Russian emergency services showed emergency medical workers sawing through the remains of the concert hall, where the seats had been charred down to their metal insides.
Reporting was contributed by Valerie Hopkins from Frankfurt, Anton Troianovski from Dubai, Oleg Matsnev from Berlin, Alina Lobzina from London, Andrew E. Kramer from Kyiv, Alan Rappeport from Washington and Victoria Kim from Seoul.