Russia observed a national day of mourning on Sunday (March 24) for the victims killed in an attack at a suburban Moscow concert hall.

Russia’s Investigative Committee said on Sunday that 137 bodies had been recovered from the Crocus City Hall, where the mass shooting took place on Friday, The New York Times reports.

“The whole country, our whole people, mourns with you,” Putin said of the country’s deadliest terrorist attack in 20 years. He also vowed to punish those responsible for the violence. “The main thing now is to prevent those who were behind this bloody massacre from committing new crimes,” he added.

The Islamic State group’s Afghanistan affiliate has taken responsibility for the deadly attack, a claim that was confirmed by U.S. intelligence officials. But Russian state news broadcasts are laying the blame on Ukraine, which is currently at war with Russia. Ukraine has denied any involvement, according to the Times.

Putin did not single out either side in his address to the country.

“We know what the threat of terrorism is,” he 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.”

On Saturday (March 23), Russian authorities arrested four men suspected of carrying out the attack at the concert hall in Krasnogorsk. Putin claimed they were captured while fleeing to Ukraine, the Associated Press reports. The Russian president said authorities detained a total of 11 people in the attack.

The company that owns Crocus City Hall has reportedly pledged to rebuild everything that was destroyed in the attack. The Times reports that the venue had more than $100 million in damages.