Shootings, explosions and a fire at a concert hall Friday evening in Russia are being investigated as a terrorist attack, the country's top investigative agency said. 

Russia’s Federal Security Service said that at least 40 people had been killed in the attack and more than 100 others are injured in one of the deadliest attacks on the country in years. 

The attack began when gunmen donning combat gear burst into a Moscow-area concert and open fired, killing at least 40 people. 

The gunmen also threw explosives inside the concert hall during the attack, which set the building on fire, Russian media reported.  

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Russia's Investigative Committee said it has opened a criminal probe into the attack but hasn't said who might be behind it. 

The attack comes on the heels of Russian President Vladimir Putin's landslide reelection. 

The Associated Press contributed to this report.