Internet threats of "dirty bomb" attacks at NFL stadiums this weekend were a hoax inspired by a writing competition between two men trying to come up with scary threats, a law enforcement official said Thursday. The threats, which were posted on a Web site last week and mentioned stadiums in seven U.S. cities, were deemed to be false by the FBI on Thursday after agents questioned a 20-year-old Milwaukee man. "This is a hoax," said Special Agent Richard Kolko, a spokesman at the FBI's Washington headquarters. A joint statement from the FBI and Homeland Security said fans "should be reassured of their security as they continue to attend sporting events this weekend."