Photos showed the Crocus City Music Hall engulfed in flames as videos emerged from the concert hall attack, showing at least four gunmen opening fire from automatic weapons as panicked Russians fled for their lives.

In one video, three men in fatigues carrying rifles fired at point blank range into bodies strewn about the lobby of the concert hall. The attackers also apparently detonated explosives, as the the sounds of blasts could be heard in other videos from the attack.

Moscow mayor Sergei Sobyanin called the attack a “great tragedy.” At least 50 ambulances had been dispatched to the site of the attack.

“The attackers presumably opened fire at the entrance to the building during a concert, using automatic weapons, and then a fire began in the building,” said emergency services. Authorities said that up to five people took part in the attack.

Russian media reports said that riot police units were sent to the area as people were being evacuated.

The shooting at the concert hall was reminiscent of some of Europe’s worst terrorist incidents such as the attack on the Bataclan in Paris in November 2015. Russians will likely recall the Nord Ost terrorist attack in Moscow in 2002, when gunmen took hostages at a theatre in one of the worst incidents in the country’s history.

The Russian news agency 112, which specialises in crime reporting, put the death toll as high as 40 killed and more than 100 injured. It was unclear if police had managed to neutralise the attackers by Friday evening, as emergency workers said they had evacuated 100 people who had been sheltering in the building’s basement.

An eyewitness told the Mash Telegram channel that there were “at least five” attackers and that they were “bearded”.

“They act like trained and trained fighters,” the account read. “At the moment of entering the building, the guards and people standing at the door were killed. Then they blocked the main entrance.”

“The terrorists are armed with AKM assault rifles,” the person said. “Some carry unloads with various ammunition. At least two of the attackers are carrying backpacks, possibly with molotov cocktails.

Russian authorities had recently carried out a series of raids against armed Islamist militants in the region of Ingushetia, leading to pitched firefights between police and the fighters.

Earlier this month, Western countries led by the United States had issued a number of warnings for their citizens not to join public gatherings in Russia. On Mar. 8, the embassy wrote it was “monitoring reports that extremists have imminent plans to target large gatherings in Moscow, to include concerts, and U.S. citizens should be advised to avoid large gatherings over the next 48 hours.”

The warnings came shortly after Russia said it had foiled a planned shooting by an Afghan group linked to Islamic State.

It was unclear if this was the attack referred to in the warning.