Russia on Sunday observed a national day of mourning in the aftermath of an attack at a suburban Moscow concert hall left more than 130 people dead. 

Family and friends of those missing were still waiting for news of their loved ones two days after the massacre, which also left some 150 people injured. The death toll, initially placed at around 40, had risen to 137 as of Sunday,  intelligence officials said.

To mark the day of mourning, 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.

“The shots were constant,” eyewitness Dave Primov told CBS News. “People panicked and started to run. Some fell down and were trampled on.” 

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. 

A United States intelligence official told CBS News that the U.S. has intelligence confirming that the Islamic State was responsible, and they do not have reason to doubt those claims. The U.S. Embassy in Russia had also previously advised Americans to stay away from concert venues because of the potential threat of a terrorist attack. The U.S. intelligence official confirmed to CBS News that the U.S. provided intelligence to Russia regarding the potential for an attack, under the intelligence community’s Duty to Warn requirement.

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

Russian authorities arrested four suspected attackers on Saturday, Putin said in a nighttime address to the nation, among 11 people 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.

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 ISIS 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.

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 U.S. warnings.

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

ISIS, 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 ISIS 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.

In October 2015, a bomb planted by ISIS downed a Russian passenger plane over Sinai, killing all 224 people on board, most of them Russian vacationers returning from Egypt.

The group, which operates mainly in Syria and Iraq but also in Afghanistan and Africa, also has claimed responsibility for several attacks in Russia’s volatile Caucasus and other regions in past years. It recruited fighters from Russia and other parts of the former Soviet Union.