LITTLETON, Colo. — A lone police cruiser outside Columbine High School was the only outward reaction Friday to an even deadlier attack at a Connecticut elementary school. But in a state that was rocked by the 1999 Columbine school massacre and the Aurora movie theater shooting less than six months ago, Friday’s shootings renewed debate over why mass shootings keep occurring and whether gun control can stop them. “Until we get our acts together and stop making these ... weapons available, this is going to keep happening,” said an angry Tom Teves, whose son Alex was killed in the theater …