MOSCOW (Reuters) -Russia lowered flags to half-mast on Sunday for a day of mourning after gunmen killed scores of people at a concert outside Moscow in the deadliest attack inside Russia for two decades and a top security official vowed vengeance.

President Vladimir Putin declared a national day of mourning after pledging to punish all those behind the attack on Friday evening, in which 137 people were killed, including three children, and 182 were injured.

Over 100 people remained in hospital, some of them in a serious condition. Video footage showed a sombre-looking Putin lighting a candle at a church at his residence outside Moscow on Sunday evening to honour those who died.

Large crowds attended a memorial event in the darkness outside the concert hall on Sunday evening and watched as projected images of white cranes – each one representing a victim of the attack – flew into the night sky as melancholy music played. Some of those present were in tears.

Islamic State has claimed responsibility for the attack, but Putin, who has yet to name those he thinks are responsible, has not publicly mentioned the Islamist militant group in connection with the attackers, who he said had been trying to escape to Ukraine. He said that some on “the Ukrainian side” had been prepared to spirit the gunmen across the border.

Margarita Simonyan, one of Russia’s top state media executives, is among those to have publicly accused Ukraine, pointing to the attackers’ lack of suicide belts and the fact that they did not plan to martyr themselves as signs that they were not genuine Islamic State militants.

Ukraine has denied any role in the attack.

Earlier on Sunday, people laid flowers at Crocus City Hall, the 6,200-seat concert hall outside Moscow where four armed men burst in 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 Islamist militants took more than 1,000 people, including hundreds of children, hostage.

Long lines formed in Moscow to donate blood as the process of identifying the dead gathered pace.

Dmitry Medvedev, deputy chairman of Russia’s Security Council, said Russia would target those behind the deadly shooting wherever they were from and whoever they were.

He had previously spoken of the need to meet “death with death” and some lawmakers have begun to discuss whether the death penalty should be re-introduced for such crimes.

Across Moscow, billboards carried a picture of a single candle, the date of the attack and the words “We mourn”. In other cities, people laid flowers.

Countries around the world have expressed horror at the attack and sent their condolences to the Russian people.

GUNMEN

Putin said 11 people had been detained, including the four suspected gunmen, who fled the concert hall and made their way to the Bryansk region, about 340 km (210 miles) southwest of Moscow.

“They tried to hide and moved towards Ukraine, where, according to preliminary data, a window was prepared for them on the Ukrainian side to cross the state border,” Putin said.

Russia’s Federal Security Service (FSB) said the gunmen had contacts in Ukraine and were captured near the border.

Blindfolded and with their hands cuffed behind their backs, the suspects were taken to the Moscow headquarters of the Investigative Committee on Sunday ahead of an expected court appearance that would remand them in pre-trial detention.

Putin ordered a full-scale invasion of Ukraine in February 2022, something he called a special military operation needed to protect Russia and Russian-speakers in Ukraine.

Ukrainian President Volodymyr Zelenskiy has accused Putin of seeking to divert blame for the concert hall attack by referring to Ukraine.

Islamic State, the Islamist group that once sought control over swathes of Iraq and Syria, claimed responsibility for the attack in a Telegram statement from the group’s Amaq agency. On Saturday night, Islamic State released on its Telegram channels what it said was footage of the attack.

In video footage published by Russian media and Telegram channels with close ties to the Kremlin, one of the suspects said he was offered money to carry out the attack.

“I shot people,” the suspect, his hands tied and his hair held by an interrogator, a black boot beneath his chin, said in poor and heavily accented Russian.

    When asked why, he said: “For money.” The man said he had been promised half a million roubles (a little over $5,000). One was shown answering questions through a Tajik translator.

Unverified footage posted to Russian Telegram channels appeared to show one of the suspects being tortured with electric shocks in detention. Reuters could not verify its authenticity.

ISLAMIC STATE

The White House said the U.S. government shared information with Russia early this month about a planned attack in Moscow, and issued a public advisory to Americans in Russia on March 7. It said Islamic State bore sole responsibility for the attack.

“There was no Ukrainian involvement whatsoever,” U.S. National Security Council spokesperson Adrienne Watson said.

Russian officials have bristled at the U.S. public comments on the attack, the first of which were made shortly after news of the attack had broken, and say Russian investigators must be allowed to make their own findings.

The United States and other Western countries whose relations with Moscow have been fraught over the Ukraine war have also sent messages of condolence to the Russian people.

If the attack was the work of Islamic State, it was unclear why the group chose this moment to strike Russia. Putin changed the course of the Syrian civil war by intervening in 2015, supporting President Bashar al-Assad against the opposition and Islamic State.