MILWAUKEE — 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."

An FBI official in Washington, who spoke on condition of anonymity because the case is still under investigation, said the Milwaukee man acknowledged posting the phony stadium threat as part of a "writing duel" with a man from the Brownsville, Texas, area to see who could post the scariest threat.

No decision has been made yet on whether charges will be filed, the official said

The threat, dated Oct. 12, appeared on a Web site, The Friend Society, that links to various online forums and off-color cartoons. Its author, identified in the message as "javness," said trucks would deliver radiological bombs Sunday to stadiums in New York, Miami, Atlanta, Seattle, Houston, Cleveland and Oakland, Calif., and that Osama bin Laden would claim responsibility.