A man remains jailed after his arrest Sunday on indecent exposure and weapons charges. Des Moines police spokesman Sergeant Todd Dykstra says an off-duty cop was using a bike trail in the capital city when he saw a man exposing himself and masturbating. The officer stopped, got off his bike, and told the man he was an off-duty policeman, then did a preliminary investigation. He used a cellphone to call the department and two officers came and arrested Mark Kevin Boston. A large knife was found in a bag Boston was carrying, and he was charged with a weapons violation and indecent exposure. He says once officers arrested him they checked the bag and found more than 100 pictures of nude children. They called detectives and after a more thorough investigation, charged him with sexual exploitation of a child. After a preliminary court appearance this (Monday) morning, Boston remained in the Polk County jail.