CHICAGO (AP) — Violent crimes — from homicides and rapes to robberies — have been on the rise in many major U.S. cities, yet experts can’t point to a single reason why and the jump isn’t enough to suggest there’s a trend. Still, it is stumping law enforcement officials, who are seeking a way to combat the problem. “It’s being reported on at local levels, but in my view, it’s not getting the attention at the national level it deserves,” FBI Director James Comey said recently.