Johnston schools opened Wednesday after a late start. Elementary and Secondary schools were on a two-hour delay to allow police to run a security sweep through the schools. It comes a day after schools were closed following a series of threatening text messages. Johnston Police Chief Dennis McDaniel says they found no credible threat and work is underway to figure out the source of the threats. Similar threats showed up in Davenport and Sioux City. Schools stayed open there. But threats against the Algona schools prompted officials to cancel classes.