A terror attack on a concert hall on the outskirts of Moscow has killed at least 115 people, Russian officials said Saturday.

Gunmen opened fire on Friday night at the 6,200-seat Crocus City Hall in the city of Krasnogorsk as people gathered for a sold-out gig by popular Russian rock band Picnic.

Initial reports in the immediate aftermath of the attack said that more than 40 people had died, but Russia’s Investigative Committee (IC) on Saturday raised the number of fatalities to 115.

The death toll could still rise as emergency efforts were still ongoing, the IC statement said.

At least three children are among the dead, according to official data.

Most of the 121 wounded were being treated in hospitals in Moscow and the surrounding region.

According to medical sources, 44 people, including two children, are in serious condition.

The assailants used “a flammable liquid” to set fire to the concert hall, the IC said. Photos showed much of the building later engulfed in flames.

The victims died of “gunshot wounds and of poisoning” from smoke inhalation, the statement added.

The Islamic State terrorist group claimed responsibility for the attack in a message on Telegram late Friday.

RIA Novosti news agency reported that one of its journalists had been at the venue and saw at least three unmasked gunmen wearing camouflage entering the hall a few minutes before 8 pm, shooting people at close range and throwing incendiary bombs.

The blaze had mostly been extinguished Saturday morning, Moscow Region Governor Andrey Vorobyov said on Telegram, adding that the roof had collapsed.

Meanwhile, the Federal Security Service (FSB) said 11 people have been arrested in connection with the attack.

FSB Director Alexander Bortnikov told Russian President Vladimir Putin that four of the alleged terrorists who took part in the attack were among the detainees. The suspects were arrested in the Bryansk region, bordering Ukraine.

According to Russian security services, the assailants intended to cross the border into Ukraine where they have «contacts».

«After the terrorist attack, the criminals intended to cross the Russian-Ukrainian border and had relevant contacts on the Ukrainian side,» the FSB said in a statement.

Ukraine said Friday that it had “nothing to do» with the attack. Russian volunteers fighting with Ukraine who recently staged several incursions into Russia also denied any involvement.

Putin has “wished all a recovery and conveyed compliments to doctors,” according to Deputy Prime Minister Tatyana Golikova, cited by state news agency Tass.

Moscow Mayor Sergei Sobyanin suspended all mass events planned for the weekend in the Russian capital, while authorities have tightened security at international airports in anticipation of further attacks.

Several nations, including Spain, Germany, Italy, China, and India have condemned the attack, while the EU said it was «shocked and appalled», adding that the bloc «condemns any attacks against civilians».

Although the White House or State Department of the United States have yet to comment, the US embassy in Moscow issued a statement on Friday night saying it was «horrified» by the reports, offering its «sincere condolences to the Russian people» and urging US citizens in the Russian capital to avoid the area.

Earlier this month, the US and Western countries had issued terror warnings and told their citizens not to attend public gatherings in Russia.

The US had shared with Russia the information it had received about an impending terrorist attack in Moscow, the White House told RIA Novosti.