WASHINGTON (Reuters) - The Department of Homeland Security has warned officials in seven U.S. cities about a dirty bomb threat to NFL football stadiums but does not believe the threat is credible, officials said on Wednesday.The threat, found posted on an Internet web site, said bombs containing radioactive material known as "dirty" bombs had been smuggled into the United States and would be used to attack professional football stadiums this coming Sunday, department spokesman Jarrod Agen said."We judge that this is not a credible threat," Agen said. He declined to identify the cities. But a department official said they were Miami, New York, Atlanta, Seattle, Houston, Oakland, California, and Cleveland, Ohio.