The attack occurred at Crocus City Hall, a popular concert venue and shopping mall in Krasnogorsk. Videos verified by The New York Times show gunmen entering the building and people wounded on the ground. Screams and gunshots can be heard in videos filmed inside the concert hall.

Moscow Mayor Sergei Sobyanin called the incident a "terrible tragedy."  Authorities have not confirmed the number of casualties, but social media reports suggest dozens were injured and up to 40 killed.

The attack follows a security alert issued by the U.S. Embassy in Moscow on March 7th, warning of potential attacks on large gatherings in the city. (ILKHA)