Spirit Lake police are investigating a series of threatening letters directed toward the local school system. Police say the 50-plus anonymous letters are offensive and harassing in nature and are directed at the Spirit Lake School girls athletic programs, other organized athletics, administrators, coaches, athletes, their parents and school board members. Over the past three months, the letters have been left on cars, in school buildings and have come through the mail. Along with the letters, there are reports of anonymous phone calls and acts of criminal mischief. People with information are asked to call the police department and they could be eligible for a 300-dollar reward.