State media reported Saturday that Russian authorities detained 11 people — including four suspected gunmen — for their involvement in a deadly attack on a crowded concert hall near Moscow on Friday that has left at least 133 people dead.
The attack left hundreds more injured, Russian officials said.
The four suspects were stopped in the Bryansk region of western Russia, "not far from the border with Ukraine," Russia's Investigative Committee said. They planned to cross the border into Ukraine and "had contacts" there, state news agency Tass said, citing Russia's FSB. The head of the FSB briefed President Vladimir Putin on the arrests on Saturday, according to Tass.
Videos circulated on Russian social media show pandemonium inside the large concert hall, which is connected to a shopping mall. Videos show people screaming and ducking for cover as gunmen fire volley after volley of automatic gunfire. Other clips show the gunmen firing, sometimes at point-blank range. The attackers also set the venue on fire, causing a partial collapse of the building's roof.
"The shots were constant," eyewitness Dave Primov told CBS News. "People panicked and started to run. Some fell down and were trampled on."
The Islamic State group has claimed responsibility for the attack. A U.S. official told CBS News that the U.S. has intelligence confirming the Islamic State's claims of responsibility, and that they have no reason to doubt those claims.
The U.S. Embassy in Russia had previously advised Americans to stay away from concert venues, citing the threat of a terrorist attack. The U.S. official confirmed that the U.S. provided intelligence to Russia about a potential attack under the intelligence community's Duty to Warn requirement.
The attack came just days after Putin cemented his grip on power in a highly orchestrated electoral landslide amid the country's war with Ukraine.
In an address to the nation, Putin called the attack "a bloody, barbaric terrorist act" and said again that all four people who were directly involved had been taken into custody. He suggested they had been trying to cross the border into Ukraine which, he said, tried to create a "window" to help them escape.
Ukraine's foreign ministry denied that the country had any involvement and accused Moscow of using the attack to try to stoke fervor for its war efforts.
"We consider such accusations to be a planned provocation by the Kremlin to further fuel anti-Ukrainian hysteria in Russian society, create conditions for increased mobilization of Russian citizens to participate in the criminal aggression against our country and discredit Ukraine in the eyes of the international community," a ministry said in a statement.
In a statement provided to CBS News Saturday, National Security Council spokesperson Adrienne Watson also rejected the idea of any involvement by Ukraine in the attack.
"In early March, the U.S. government shared information with Russia about a planned terrorist attack in Moscow," Watson said. "We also issued a public advisory to Americans in Russia on March 7. ISIS bears sole responsibility for this attack. There was no Ukrainian involvement whatsoever."
Ukrainian President Volodymyr Zelenskyy angrily rejected Moscow's accusations as an attempt by Putin and his lieutenants to shift the blame to Ukraine while treating their own people as "expendables."
"They are burning our cities — and they are trying to blame Ukraine," he said in a statement on his messaging app channel. "They torture and rape our people — and they blame them. They drove hundreds of thousands of their terrorists here to fight us on our Ukrainian soil, and they don't care what happens inside their own country."
Images shared by Russian state media showed emergency vehicles still gathered outside the ruins of the concert hall, which could hold more than 6,000 people and hosted many big events, including the 2013 Miss Universe beauty pageant that featured Donald Trump.
Secretary of State Antony Blinken also said in a statement that the U.S. "strongly condemns" the deadly attack.
"We send our deepest condolences to the families and loved ones of those killed and all affected by this heinous crime," Blinken said. "We condemn terrorism in all its forms and stand in solidarity with the people of Russia in grieving the loss of life from this horrific event."
A U.S. law enforcement official told CBS News that there is no known threat to the U.S. emanating from the Moscow attack.
Meanwhile, in Moscow, hundreds of people stood in line Saturday morning to donate blood and plasma, Russia's health ministry said.
This is the most deadly terror attack in Russia in years. The country was shaken by a series of deadly terror attacks in the early 2000s during the fighting with separatists in the Russian province of Chechnya.
In October 2002, Chechen militants took about 800 people hostage at a Moscow theater. Two days later, Russian special forces stormed the building, and 129 hostages and 41 Chechen fighters died, most of them from the effects of narcotic gas Russian forces used to subdue the attackers.
And in September 2004, about 30 Chechen militants seized a school in Beslan in southern Russia, taking hundreds of hostages. The siege ended in a bloodbath two days later and more than 330 people, about half of them children, were killed.
— CBS News' Debora Patta, David Martin, Andy Triay and Olivia Gazis contributed to this report.