After one of the most serious terrorist attacks in Russian history, in which gunmen fired indiscriminately at concert-goers at a venue on the outskirts of Moscow, killing 133 people, many questions remain. Another 152 people were injured, with many of them remaining in critical condition on Sunday, according to a report from the state news agency TASS, which cited the Moscow regional Civil Protection. Five children are reportedly among the injured. Russia observed a national day of mourning on Sunday. The Islamic State (IS) militia claimed responsibility for the deadly attack on the concert ha…