At least 133 people were killed and more than 140 injured when gunmen attacked a packed concert venue on the outskirts of Moscow, Russia saysA large fire engulfed the roof of the complex and dramatic video shows panicked concertgoers taking cover as shots and explosions ring outRussian President Vladimir Putin says all four gunmen have been arrested, and that the suspects were trying to flee to Ukraine - Kyiv says allegations of Ukrainian involvement are "absurd"Putin calls the attack a "barbaric terrorist act" and announces a day of national mourning for 24 MarchMuscovites are queuing to give blood for those injured and flower tributes have been placed at the scene of the attackThe US says it's credible that the Islamic State group could be behind the attack, after the group said it did it. Russia has not commentedBBC