Russia said Saturday that it had detained 11 people, including four of the suspected shooters, in the mass shooting at a concert hall outside Moscow and increased the death toll to 115, according to reports. 

The Islamic State terror group has claimed responsibility for the Friday rampage at the Crocus City Hall on the outskirts of the Russian capital, the deadliest attack in Russia in 20 years, even as the Kremlin pointed blame at Ukraine, the Associated Press reported.

The four suspected gunmen were nabbed in western Russia "not far from the border with Ukraine," Russia's investigative committee announced, the AP reported.

The four planned to cross into Ukraine, which Russia invaded in Feb. 2022, and "had contacts" in the country, Russia's state news agency Tass reported, citing Russia's FSB security agency.

The chief of the FSB has briefed President Vladimir Putin on the arrests.

Mykhailo Podolyak, an aide to Ukrainian President Volodymyr Zelensky, denied any involvement.

"Ukraine has never resorted to the use of terrorist methods," he posted on social media, the AP reported. "Everything in this war will be decided only on the battlefield."

A U.S. intelligence official told the AP that U.S. agencies confirmed ISIS carried out the attack. 

In a statement posted by its Aamaq news agency, the ISIS affiliate in Afghanistan said it carried out an attack on "Christians" in Russia.

The AP could not verify the claim.

Gunmen, dressed in camouflage and carrying automatic weapons, were seen on videos posted online shooting people gathered to see the Russian rock band Picnic at point-blank range and tossing incendiary devices into the crowd, reports said.

The theater's roof collapsed early Saturday even as firefighters battled the flames ignited in the attack.

Video images showed concert-goers taking their seats as gunfire suddenly reverberated throughout the venue and chaos broke out with people fleeing in a panic.

The gunmen fired into the crowds of people, mowing them down, leaving some to play dead in pools of blood, Reuters reported.

"Suddenly there were bangs behind us - shots. A burst of firing - I do not know what," one person told Reuters.

A Russian news outlet reported that law enforcement agencies found 28 bodies in a bathroom and 14 on a staircase, Reuters reported. 

"Many mothers were found embracing their children," the report said.