Russia has lowered flags to half-mast for a day of mourning after scores of people were gunned down with automatic weapons at a rock concert outside Moscow in the deadliest attack inside Russia for two decades.

President Vladimir Putin declared a national day of mourning for Sunday after pledging to track down and punish all those behind the attack, in which at least 137 people were killed, including three children, and more than 150 injured.

“I express my deep, sincere condolences to all those who lost their loved ones,” Putin said in an address to the nation on Saturday, his first public comments on the attack. “The whole country and our entire people are grieving with you.”

The ISIL (ISIS) armed group claimed responsibility for Friday’s attack, but Putin has not publicly mentioned the group in connection with the attackers, whom he said had been trying to escape to Ukraine. He asserted that some on “the Ukrainian side” had prepared to spirit them across the border.

Ukraine has repeatedly denied any role in the attack, which Putin also blamed on “international terrorism”.

People on Sunday laid flowers at Crocus City Hall, the 6,200-seat concert hall outside Moscow where four armed men burst in on Friday just before Soviet-era rock group Picnic was to perform its hit, Afraid of Nothing.

The men fired their automatic weapons in short bursts at terrified civilians who fell screaming in a hail of bullets.

It was the deadliest attack on Russian territory since the 2004 Beslan school siege, when attackers took more than 1,000 people, including hundreds of children, hostage.

Governor of the Moscow region Andrei Vorobyov said on Sunday that the rescue operation was completed and the search operation is still ongoing.

“Identification by relatives is ahead. In hospitals, doctors are fighting for the lives of 107 people,” he said.