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 linked to a Muslim group 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.

Al Jazeera’s Yulia Shapovalova, reporting from outside the concert hall, said people have been bringing flowers, candles, stuffed animals and posters for a makeshift memorial outside the hall.

“We can see flags raised to half-staff on the Russian house of parliament and other buildings. People are shocked, they’re grieving … there are numerous memorials throughout Russia,” she said.

“The clearance of the rubble continues with rescue dogs looking for people under the rubble … the death toll could rise.”

Long lines formed in Moscow to donate blood. Blood banks said on Sunday that they now had enough blood supplies for four to six months.

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

Following a Palm Sunday mass at St Peter’s Square in Vatican City, Pope Francis sent prayers to victims of the attack.

“I assure my prayers for the victims of the cowardly terrorist attack carried out the other evening in Moscow,” the 87-year-old pope said.

Headed for Ukraine

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

Meduza, a Russian publication based in Latvia, confirmed the claims by Russian state officials that the suspects were headed for Ukraine, by geolocating the filming location of the reported arrest of one of the suspects.

According to footage provided by the FSB, officials arrested a 30-year-old near the town of Khatsun in Russia’s Bryansk region, located 14km (8.6 miles) from the Ukrainian border, Meduza reported on Saturday.

The suspects have been brought to Moscow and may appear in court later in the day, according to local news agencies.

Ukrainian President Volodymyr Zelenskyy said it was typical of Putin and “other thugs” to seek to divert blame.

ISIL, which once sought control over swaths of Iraq and Syria, claimed responsibility for the attack, the group’s Amaq News Agency said on Telegram.

On Saturday night, ISIL released on its Telegram channels what it said was footage of the attack.

In 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 highly accented Russian.

When asked why, he said: “For money.” The man said he had been promised half a million roubles ($5,400).

One was shown answering questions through a Tajik translator.

ISIL behind attack?

The White House said the United States 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 ISIL bore sole responsibility for the attack.

“There was no Ukrainian involvement whatsoever,” US National Security Council spokeswoman Adrienne Watson said.

Russia’s Foreign Ministry spokeswoman Maria Zakharova said on Sunday that any statement made by US authorities to vindicate Kyiv until the end of the probe into the attack should be considered as evidence.

Russian Ambassador to the US Anatoly Antonov told the TASS news agency on Saturday that the US did not pass any specific information through the Russian embassy in Washington about preparations for the attack.

“Nothing was passed,” the ambassador said. “No concrete information, nothing was transferred to us.”