Russia observed a national day of mourning on Sunday after a massacre in a Moscow concert hall killed 137 people, the deadliest attack in Europe to have been claimed by the Islamic State jihadist group.President Vladimir Putin has vowed to punish those behind the “barbaric terrorist attack”, saying four gunmen trying to flee to Ukraine had been arrested.Kyiv has strongly denied any connection to the attack, with President Volodymyr Zelensky accusing Putin of trying to shift the blame.”The whole country is in mourning with those who lost their loved ones in this inhumane tragedy,” public television channel Russia 24 said on Sunday morning.Putin, in his only public remarks on the attack, made no reference to IS’s claims of responsibility.At least 137 people, including three children, were killed when camouflaged gunmen stormed the Crocus City Hall, in Moscow’s northern suburb of Krasnogorsk, and then set fire to the building on Friday evening.Russia’s Investigative Committee also said Sunday that guns and rounds of ammunition had been found both there and in a car that was used by the suspected gunmen to flee the scene.The agency posted a video of the four suspects being dragged into its headquarters in Moscow. There was no statement on the other seven suspects arrested in connection with the attack.Officials have not named the shooters, but said they were all foreign nationals.- ‘Machine guns, knives, firebombs’ -The Islamic State group posted Saturday on Telegram that the attack was “carried out by four IS fighters armed with machine guns, a pistol, knives and firebombs” as part of “the raging war” with “countries fighting Islam”.A video lasting about a minute and half, apparently filmed by the gunmen, has been posted on social media accounts typically used by IS, according to the SITE intelligence group.The video, which appears to have been filmed from the lobby of the concert venue, shows several individuals with blurred faces and garbled voices, firing assault rifles with inert bodies strewn on the floor and a fire starting in the background.Russian investigators said that after walking through the theatre shooting spectators, they set fire to the building, trapping many inside who died from smoke inhalation.Russian officials expect the death toll to rise further, with 110 people still in hospital and 40 in “critical” or “extremely critical” condition. The attack was the deadliest in Russia since the Beslan school siege in 2004.The emergency situations ministry has so far named 29 of the victims, the blaze having complicated the process of identification. The ministry on Sunday posted a video of heavy equipment arriving at the venue to dismantle damaged structures and clear debris.- ‘Morally crushed’ -On the streets of the capital on Sunday, there was shock and grief.”It is a tragedy. I was morally crushed,” Ruslana Baranovskaya, 35, told AFP.”People don’t smile… everybody feels the loss,” said 73-year-old Valentina Karenina, a pensioner standing on a street off Red Square.Museums, theatres and cinemas around the country closed and billboards were replaced with memorial posters. Mourners continued to stream to the concert hall in northwest Moscow to lay flowers at a tribute to the victims.More than 5,000 people donated blood following the attack, officials said, with many standing in long queues outside clinics.Putin on Saturday vowed “retribution and oblivion” to the “terrorists, murderers and non-humans” who carried out the “barbaric terrorist attack”.Several of his allies have called for the country to lift a moratorium on the death penalty, sparking concern among Kremlin critics.- Putin points to Ukraine -Putin has pointed to a Ukraine connection and has not publicly addressed IS’s claim of responsibility.”They tried to escape and were travelling towards Ukraine, where, according to preliminary data, a window was prepared for them on the Ukrainian side to cross the state border,” Putin said of the four attackers in a televised address to the nation on Saturday, his only public comments so far.Zelensky, in his own evening address Saturday, rejected the suggestion that Kyiv had been involved. “Putin and the other scum are just trying to blame it on someone else,” he said.  In Moscow, some doubted Putin’s claims that Ukraine was involved.”I’m not inclined to the version about Ukraine’s involvement… this is more like those committed by Islamist extremists,” said Vomik Aliyev, a 22-year-old who often went to the concert hall and who said his parents were Muslim.Washington on Sunday again rejected that Kyiv was involved in the attack.”ISIS bears sole responsibility for this attack,” said White House National Security Council spokeswoman Adrienne Watson. “There was no Ukrainian involvement whatsoever.” bur/js