Two days after one of the most serious terrorist attacks in Russian history, the Islamic State extremist militia published a video of Friday’s mass shooting at a concert hall on the outskirts of Moscow in which at least 137 people were killed. Four unidentified gunmen fired indiscriminately at concert-goers at the Crocus City Hall venue in north-west Moscow shortly ahead of a concert on Friday evening. Eyewitnesses also reported that they heard explosions in the building before a major fire broke out. Islamic State claimed responsibility for the deadly attack but the Russian authorities have y…