Police are investigating after someone called in a threat to Harrisburg International Airport.HIA first alerted the public about the threat around 11:30 p.m. Thursday on Twitter.The tweet said that law enforcement had evacuated the terminal as a precaution and were still searching the building.Planes arriving at the airport were held on the ramp.HIA gave the all-clear just after 1 a.m. Friday, stating:"Law enforcement officials have cleared the terminal at @FlyHIA. Nothing found and normal operations have resumed. Aircraft now coming to the gates."Stay with WGAL for updates on this developing story.