Death toll rises to 133, suspects in custody in Moscow music hall attack. Here's what we know.

The death toll rose Saturday to at least 133 and a number of suspects were in custody, according to Russian officials, after gunmen stormed a Moscow concert hall Friday in the deadliest attack in the capital city in years.

The militant group ISIS claimed responsibility for the attack Friday night on social media. More than 140 people were also injured in the onslaught, in which numerous gunmen armed with rifles entered the hall before a concert, opening fire on attendees and setting the premises ablaze. Here's what we know.

What happened

Friday's shooting occurred at Crocus City Hall in Krasnogorsk, west of central Moscow. RIA Novosti, a Russian state-run news agency, reported that assailants in camouflage "burst into the ground floor” of the venue and “opened fire with automatic weapons.”

Videos posted to social media and confirmed by U.S. news outlets showed men with rifles moving through the concert hall, firing on those in attendance. Photos and videos from the scene also showed the concert hall engulfed in flames.

According to NBC News, the Russian progressive rock band Picnic was scheduled to perform a sold-out show at the 9,500-seat venue on Friday. According to officials, the shooting occurred before the start of the show.

The gunmen escaped after the attacks, but Russian authorities later said they had detained 11 people, including the four suspects they believe responsible for the shooting.

Who is Islamic State?

The Afghanistan branch of ISIS, which the United States has designated as a terrorist group, said in a message posted Friday night to the Telegram messaging app that it had “attacked a large gathering… on the outskirts of the Russian capital Moscow," and that the attackers had "retreated to their bases safely," according to the Moscow Times. A U.S. intelligence official confirmed to the Associated Press that the group had orchestrated the attack.

Primarily based in Iraq and Syria, the Islamic State and its offshoots are a Sunni jihadist group. They also operate also in Afghanistan and Africa and have been known to recruit fighters from Russia.

What are authorities saying?

President Vladimir Putin, addressing the country Saturday, attempted to link the attacks to Ukraine, which has been under Russian invasion since February 2022. Putin said the alleged assailants had been attempting to flee to Ukraine, without providing any evidence.

Ukraine, however, denied any involvement in the attacks.

"This is of course another lie from the Russian special services, which has nothing in common with reality and does not stand up against any criticism," a Ukrainian spokesperson told the Reuters news agency.

U.S. had warned of possible ‘extremist’ attacks

Earlier this month, the U.S. Embassy in Moscow issued a security alert warning that it was “monitoring reports that extremists have imminent plans to target large gatherings in Moscow,” including concerts.

The March 7 statement advised U.S. citizens in the Moscow area to “avoid large gatherings over the next 48 hours.”

According to Reuters, the embassy issued its warning several hours after Russia's Federal Security Service (FSB) said it “foiled an attack on a synagogue in Moscow by a cell of the militant Sunni Muslim group Islamic State.”

Shooting comes on heels of Putin's reelection

Friday's deadly attack comes less than a week after Putin won a fifth term in a landslide election that U.S. and other Western leaders denounced as a sham.

Speaking Saturday for the first time on the tragedy, Putin vowed to punish those behind the attack, and said security measures had been tightened up across Russia. He also announced that Sunday would be a day of mourning for the nation.