A claim of responsibility by Islamic State for a massacre of Russian concertgoers near Moscow appears to be plausible and fits with a pattern of previous marauding attacks by Islamist militants, security analysts said on Saturday. One leading expert said, however, it was unusual and striking that the assailants had formed and executed an escape plan instead of pursuing their rampage to the point of being gunned down. Islamic State, also known as IS or ISIS, issued statements claiming responsibility for Friday evening's attack in which Russian investigators said 133 people were killed, a...