The death toll in Friday’s terror attack on a concert in Moscow has soared to at least 133, Russia’s top state investigative agency said Saturday, with authorities saying they have apprehended 11 suspects, four of whom were directly involved in the onslaught.

President Putin addressed the shocked nation in a televised broadcast on Saturday, slamming the massacre as "a bloody, barbaric terrorist act." He declared Sun., March 24, a day of mourning.

Putin said that additional security measures have been put in place throughout the country following the attack – the deadliest in over 20 years.

"All four direct perpetrators of the terrorist attack, all those who shot and killed people, were found and detained," Putin said. "They tried to hide and moved towards Ukraine, where, according to preliminary data, a window was prepared for them from the Ukrainian side to cross the state border."

RUSSIA INVESTIGATING SHOOTING, EXPLOSIONS AT CONCERT HALL AS TERRORIST ATTACK

The Islamic State of Iraq and Syria (ISIS) claimed responsibility for the deadly attack in a statement on its affiliated social media channels, and the U.S. has said it has intelligence confirming that, according to several outlets. Russia's Investigative Committee said it has opened a criminal probe into the attack.

However, Putin suggested there may have been a Ukrainian link to the bloodbath. Ukrainian President Volodymyr Zelenskyy has denied any involvement by his nation.

The surprise attack began when gunmen donning combat gear burst into Crocus City Hall in the city of Krasnogorsk, where concertgoers were gathering to hear the Russian band Picnic. Video online showed gunmen opening fire, shooting attendees at close range and setting off smoke bombs. 

The gunmen also threw explosives inside the concert hall during the attack, rocking the building and setting it on fire, Russian media reported. People were being evacuated, but some remained trapped inside the burning building, Russian media said. 

The roof of the theater collapsed in the early hours of Saturday morning as firefighters spent hours fighting the flames. Russians laid flowers at memorials and lined up to give blood.

Putin said that the Federal Security Service (FSB) of Russia and other law enforcement agencies are working to identify the entire terrorist support base.

"Those who provided them with transport, planned escape routes from the crime scene, prepared caches, caches of weapons and ammunition," Putin said. 

"It is already obvious that we are faced not just with a carefully and cynically planned terrorist attack, but with the organized mass murder of peaceful, defenseless people. The criminals were cold-blooded and purposefully going to kill, shoot our citizens at point-blank range - our children."

The FSB said the attack has been carefully planned. 

"The weapons that the terrorists used had been placed in a cache in advance," the FSB noted.

The FSB had been pinning the blame on Ukraine, with Russia's Investigative Committee saying that four suspects were stopped in the Bryansk region of western Russia, "not far from the border with Ukraine." 

RUSSIA INVESTIGATING SHOOTING, EXPLOSIONS AT CONCERT HALL AS TERROR ATTACK

They planned to cross the border into Ukraine and "had contacts" there, state news agency Tass said, citing Russia’s FSB. 

Ukrainian President Volodymyr Zelenskyy denied any Ukrainian involvement before ISIS claimed responsibility. 

"Ukraine certainly has nothing to do with the shooting/explosions in the Crocus City Hall (Moscow Region, Russia). It makes no sense whatsoever," he said on social media, adding, in part, "there is not the slightest doubt that the events in the Moscow suburbs will contribute to a sharp increase in military propaganda, accelerated militarization, expanded mobilization, and, ultimately, the scaling up of the war. And also to justify manifest genocidal strikes against the civilian population of Ukraine." 

The attack came just days after Russian President Vladimir Putin cemented his grip on power in another electoral landslide. The attack was the deadliest in Russia in years and came as the country’s fight in Ukraine dragged into a third year.

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Putin has wished all those injured a speedy recovery and praised medical staff, Deputy Prime Minister Tatyana Golikova said.

"We together with [Russian Health Minister] Mikhail A. [Murashko] reported to the president on the state of health of patients, the president wished all a recovery and conveyed compliments to doctors," she told reporters.

Russian special services are still conducting search operations. Moscow Mayor Sergey Sobyanin canceled all mass events in the capital in the next two days, and a number of other Russian regions followed suit.

Fox News’ Brie Stimson as well as The Associated Press and Reuters contributed to this report.