Several gunmen opened fire inside a large concert hall in Moscow, Russia, on Friday. The number of people killed or injured in the incident remains unclear.

Russian state media reported that three people dressed in camouflage used automatic weapons and opened fire in the Crocus City Hall. There were multiple explosions during the chaotic scene.

The Federal Security Service said there were fatalities but did not specify the number of victims.

Russian Foreign Ministry spokeswoman Maria Zakharova described it as a terrorist attack.

Moscow Mayor Sergei Sobyanin called the incident a “terrible tragedy.”

“Today a terrible tragedy occurred in the Crocus City center. My condolences to the loved ones of the victims. I gave orders to provide all necessary assistance to everyone who suffered during the incident,” Sobyanin said in a statement.

The attack occurred about two weeks after the U.S. Embassy in Russia warned U.S. citizens to avoid large gatherings due to concerns that “extremists have imminent plans” to target large gatherings in Moscow. There’s no clear connection between the attack and the previous warning from the embassy.

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The warning, issued on March 7, stressed that the threat was most concerning for the 48 hours after it was alerted, which has long since passed.

This is a developing story. Check back for updates.