Hoax calls involving alleged bomb threats and even claims of active shooters have prompted evacuations and closures at several zoos around the U.S. in recent days. No explosives or real dangers have been found in the latest string of what authorities are describing as “swatting” incidents. In the latest case, police on Sunday swept the Akron Zoo in Ohio after a threat led to the evacuation of visitors. Threats also have been called in to zoos elsewhere in Ohio, as well as in Kentucky, Tennessee, Florida and Arizona. The FBI has called swatting an increasing national problem.