How did the Moscow attack happen?

Security has been one of the central pillars of Vladimir Putin's presidency and the embarrassment felt by this attack means it could have ramifications that spread beyond Russia's borders. But how did it happen? The target was an entertainment complex on the outskirts of Moscow in the northwest suburb of Krasnogorsk. It was just before eight o'clock on Friday night and the auditorium at Krokus City Hall was filling up. People had come to see a rock concert by a Soviet era band called Picnic. We have a satellite image of the overall complex and we've marked on where it's likely the attackers entered on the right-hand side here. We'll come to how they moved through the arena shortly. But first let's look at the floor plans of the auditorium itself. It has three tiers, a capacity of 6,200 and this event was sold out. Not everyone was in their seats when the attack began. The concert hadn't started and the first victims were actually targeted outside in the foyer. Now together with videos taken on the ground we can see the attackers route. The gunman entered the foyer in the bottom corner there and they immediately started shooting. We then see them crossing the striped marble marble floor as they head towards the auditorium and the next video shows them moving together spraying bullets as they go. Although we don't know their exact route we then do know the attackers made their way into the main concert hall and it's here we see them moving between the rows of seats shooting victims at point-blank range. Today the governor of the Moscow region visited the site with investigators. He was filmed walking through into the stalls section of the theatre and it's in this video we really understand the true scale of the destruction. It's laid bare. The fire that engulfed the complex and caused some of the roof to collapse has left a burnt-out shell. Russia says it detained four suspects in the Bryansk region of the country which is around four hours away from Moscow. It also borders Belarus and Ukraine. We also have an image of what is reportedly the car the suspects were traveling in when they were stopped. The Kremlin claims they were heading to Ukraine hoping to cross the border but Kiev has denied any involvement in the attack. So while we can tell you how the attack unfolded the question of who's to blame is disputed.