LONDON, Ont.—A family of five Muslims out for an evening early summer stroll were mowed down by a driver in an “act of mass murder,” the mayor of London, Ont., said on Monday. The horrific incident on Sunday evening left four of them dead and a boy with serious injuries, police said. “Words fail on a day as dark as this but words matter,” Mayor Ed Holder said. “This was an act of mass murder perpetrated against Muslims, against Londoners, and rooted in unspeakable hatred.” A 20-year-old city man was arrested in the parking lot of a mall seven kilometres away. He now faces four counts of first-degree murder and one of attempted murder, police said. Police Chief Steve Williams said relatives had asked no names be released but identified the four victims as a 74-year-old woman, a 46-year-old man, a 44-year-old woman and a 15-year-old girl. A nine-year-old boy …