Moscow. Family and friends of those still missing after an attack that killed more than 130 people at a suburban Moscow concert hall waited for news of their loved ones as Russia observed a day of national mourning on Sunday.

Events at cultural institutions were canceled, flags were lowered to half-staff and television entertainment and advertising were suspended, according to state news agency RIA Novosti. A steady stream of people added to a makeshift memorial near the burnt-out concert hall, creating a huge mound of flowers.

“People came to a concert, some people came to relax with their families, and any one of us could have been in that situation. And I want to express my condolences to all the families that were affected here and I want to pay tribute to these people," Andrey Kondakov, one of the mourners who came to lay flowers at the memorial, told The Associated Press.

“It is a tragedy that has affected our entire country," kindergarten employee Marina Korshunova said. “It just doesn’t even make sense that small children were affected by this event.” Three children were among the dead.

The attack, which has been claimed by an affiliate of the Islamic State group, is the deadliest on Russian soil in years.

As rescuers continue to search the damaged building and the death toll rises as more bodies are found, some families still don’t know if relatives who went to the event targeted by gunmen on Friday are alive. Moscow's Department of Health said Sunday it has begun identifying the bodies of those killed via DNA testing, which will need at least two weeks.

Igor Pogadaev was desperately seeking any details of his wife’s whereabouts after she went to the concert and stopped responding to his messages.

He hasn't seen a message from Yana Pogadaeva since she sent her husband two photos from the Crocus City Hall music venue.

After Pogadaev saw the reports of gunmen opening fire on concertgoers, he rushed to the site, but couldn’t find her in the numerous ambulances or among the hundreds of people who had made their way out of the venue.

“I went around, searched, I asked everyone, I showed photographs. No one saw anything, no one could say anything,” Pogadaev told the AP in a video message.

He watched flames bursting out of the building as he made frantic calls to a hotline for relatives of the victims, but received no information.

As the death toll mounted on Saturday, Pogodaev scoured hospitals in the Russian capital and the Moscow region, looking for information on newly admitted patients.

But his wife wasn’t among the 154 reported injured, nor on the list of 50 victims authorities have already identified, he said.

Refusing to believe that his wife could be one of the 137 people who died in the attack, Pogadaev still hasn't gone home.

“I couldn’t be alone anymore, it’s very difficult, so I drove to my friend’s,” he said. “Now at least I’ll be with someone.”

The Moscow Region’s Emergency Situations Ministry posted a video Sunday showing equipment dismantling the damaged music venue to give rescuers access.

Meanwhile, President Vladimir Putin appears to be trying to tie Ukraine to the attack, something its government firmly denies.

He was filmed Sunday lighting candles in memory of the victims at Novo-Ogaryovo, one of Russia's presidential residences just outside Moscow.

Russian authorities arrested four suspected attackers on Saturday, Putin said in a nighttime address to the nation, among 11 people detained suspicion of involvement in the attack. He said that they were captured while fleeing to Ukraine.

Though no court hearing has been officially announced, there was a heavy police presence around Moscow’s Basmanny District Court on Sunday. Police tried to drive journalists away from the court.

Footage released by Russia's Investigative Committee showed the suspected attackers being dragged while blindfolded into the state body's headquarters in Moscow.

Putin called the attack “a bloody, barbaric terrorist act” and said Russian authorities captured the four suspects 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.

Kyiv strongly denied any involvement, and the Islamic State group’s Afghanistan affiliate claimed responsibility.

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

US intelligence officials said they had confirmed the IS affiliate's claim.

“ISIS bears sole responsibility for this attack. There was no Ukrainian involvement whatsoever,” National Security Council spokeswoman Adrienne Watson said in a statement.

The US shared information with Russia in early March about a planned terrorist attack in Moscow, and issued a public warning to Americans in Russia, Watson said.

The raid was 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 US warnings.

White House press secretary Karine Jean-Pierre said in a statement that the US condemned the attack and said that the Islamic State group is a “common terrorist enemy that must be defeated everywhere.”

IS, which fought against Russia during its intervention in the Syrian civil war, has long targeted Russia. In a statement posted by the group’s Aamaq news agency, the IS Afghanistan affiliate said that it had attacked a large gathering of “Christians” in Krasnogorsk.

The group issued a new statement Saturday 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 the Islamic State group's ongoing war with countries that it says are fighting against Islam.