Gunmen wearing camouflage burst into a concert venue on the outskirts of Moscow on Friday, killing at least 133 people and injuring 145 others. A fire broke out with flames visible from miles away.
Russia's Investigative Committee, which released the number of casualties, said in a statement that "unfortunately, the number of victims could increase."
Eleven people were detained, including four who were directly involved in the attack, according to a statement from the Russian Federal Security Service, or FSB.
The agency said the four were arrested in the Bryansk region, and were attempting to drive across the Russian-Ukraine border.
The U.S. believes a group known as Islamic State Khorasan Province, or ISIS-K, is responsible, an administration official told NPR. The group, based in eastern Afghanistan, had claimed responsibility on its Telegram channel.
The U.S. warned Russia earlier this month that it had information about a planned terror attack in Moscow, potentially targeting concerts, said National Security Council spokeswoman Adrienne Watson in a statement.
Concertgoers had gathered at Moscow's Crocus City Hall venue to see the veteran Russian rock band Picnic when several gunmen in military fatigues entered.
Witness videos on social media show at least three attackers firing at bystanders as they attempted to hide. Other images show people lying on the ground in the main concert hall before a fire breaks out and spreads, trapping some inside. Several people were earlier spotted on the venue's roof, which later appeared to collapse from the fire.
Moscow Mayor Sergey Sobyanin said "a huge tragedy has occurred" as the number of dead is expected to rise, and expressed his condolences.
Russian President Vladimir Putin has yet to comment directly on events, instead wishing a speedy recovery to the injured and expressing thanks to doctors through a senior administration official.
The Kremlin said Putin was informed of the attack minutes after it began and received reports from the heads of his security and emergency situations ministries overnight.
Some Russian officials are suggesting there must be some Ukrainian role, a claim the Ukrainians deny. Mykhailo Podolyak, an advisor to Ukrainian President Volodymyr Zelenskyy tweeted "Ukraine certainly had nothing to do with the shooting in the Crocus City Hall. It makes no sense whatsoever."
The White House issued a statement calling the attack "terrible" and saying there was "no indication at this time" of a Ukrainian role in the attack.
National Security Council spokesman John Kirby said the images of the shooting were "horrible and hard to watch." The U.S. embassy in Moscow has warned U.S. citizens to avoid the area.
Moscow was the scene of a hostage crisis at the Dubrovka theater in 2002, in which Chechen gunmen demanding the withdrawal of Russian troops from Chechnya took some 800 theatergoers hostage. A raid by Russian security forces to end the crisis killed or caused the deaths of 172 people.
Russian Foreign Ministry spokesperson Maria Zakharova called on the international community to condemn the incident, calling it "a monstrous crime."
Meanwhile, in Moscow and Saint Petersburg, people lined up to donate blood for victims.
Impromptu flower memorials have sprung up in other Russian cities, as well as outside Russian embassies in other countries, including at the Russian consulate in New York.
This is a breaking news story and will be updated.
