A Web site is claiming that seven professional football stadiums will be hit with radiological dirty bombs this weekend, but the U.S. government on Wednesday expressed doubts about the threat.

The warning, posted Oct. 12, was part of a continuing Internet conversation titled "New Attack on America Be Afraid." It mentioned National Football League stadiums in New York, Miami, Atlanta, Seattle, Houston, Oakland and Cleveland, where games are scheduled for this weekend.

The Homeland Security Department alerted authorities and stadium owners in those cities, as well as the NFL, of the Web message but said the threat was being viewed "with strong skepticism." Homeland Security spokesman Russ Knocke said there was no intelligence that indicated such an attack was imminent, and he said the alert was "out of an abundance of caution."

"The department strongly encourages the public to continue to go about their plans, including attending events that involve large public gatherings such as football games," Knocke said.

The Federal Bureau of Investigation also expressed doubt about the threat.

The U.S. alert level remains at yellow, signaling an elevated risk of an attack. The threat level for airline flights is at orange, a higher level, where it has been since a foiled plot to bomb U.S.-bound commercial jets was revealed on Aug. 10.

NFL spokesman Greg Aiello said stadiums around the United States "are very well protected through the comprehensive security procedures we have in place, including secure facility perimeters, pat-downs and bag searches."

Officials were made aware of the Web posting on Oct. 16. The threat was timed to be carried out on Sunday, Oct. 22, marking the final day in Mecca of Ramadan, the Islamic holy month.

"The death toll will approach 100,000 from the initial blasts and countless other fatalities will later occur as result from radioactive fallout," according to a copy of the posting that was obtained by The Associated Press.

The bombs, according to the posting, would be delivered to the stadiums in trucks. All but one of the stadiums — Atlanta — are open-air arenas, the posting noted, adding: "Due to the open air, the radiological fallout will destroy those not killed in the initial explosion."

Explosions would be nearly simultaneous, the posting said, with the cities specifically chosen in different time zones.

The posting said that Al Qaeda would automatically be blamed for the attacks and predicted, "Later, through al-Jazeera, Usama bin Laden will issue a video message claiming responsibility for what he dubbed 'America's Hiroshima."'