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, said.

“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 an nighttime address to the nation, among 11 people who were detained on 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.

A woman grieves at a makeshift memorial in front of the Crocus City Hall in Krasnogorsk on Sunday, as Russia observes a national day of mourning after a massacre that killed more than 130 people. (Photo by Olga Maltseva/AFP via Getty Images)