WASHINGTON – A Web site is claiming that seven NFL football stadia will be hit with radiological dirty bombs this weekend, but the government on Wednesday expressed doubts of the threat.

The warning, posted Oct. 12, was part of an ongoing Internet conversation titled “New Attack on America Be Afraid.” It mentioned NFL stadia in New York, Miami, Atlanta, Seattle, Houston, Oakland and Cleveland, where games are to be played Sunday.

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.” Officials at the NCAA, which oversees college athletics, said they too had been notified.

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 FBI also expressed doubt about the threat.

“While the credibility of the threat is questionable, we have passed the information on because it has been carried in some open source reporting,” said FBI spokesman Richard Kolko. He said the FBI was discussing the threat with the NFL as “part of our routine discussions this week.”

The nation’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 stadia around the country “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 stadia in trucks. All but one of the stadia – 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-Qaida would be blamed for the attacks and predicted, “Later, through al-Jazeera, Osama bin Laden will issue a video message claiming responsibility for what he dubbed ‘America’s Hiroshima.'”

Tony Wyllie, the vice president of communications for the Houston Texans, said the team had been in contact with the NFL regarding what security precautions should be taken for Sunday’s game against the Jaguars.

In Indianapolis, where the Colts were preparing for a home game this weekend, head coach Tony Dungy said, “I’ve been waiting for this to happen for a couple of years now and you try and handle the security and put it out of your mind.”

“We’ll let the security people do their job, and we’ll do our job,” Dungy said. “We’ve got a lot of confidence in NFL security and our own security here.”

Amy Trask, CEO of the Oakland Raiders, said, “We work closely with a number of governmental agencies, including the FBI, and with the NFL on an ongoing basis.”

Associated Press writers Dave Goldberg and Ralph Russo in New York, Kristie Rieken in Houston, Josh Dubow in Alameda, Calif., and Michael Marot in Indianapolis contributed to this report.

Amy Trask, CEO of the Oakland Raiders, said, “We work closely with a number of governmental agencies, including the FBI, and with the NFL on an ongoing basis.”

Stacey Osburn, associate director of public relations for the NCAA, said the organization passed the warning to members “so that they may take the appropriate precautions.”

The postings were made on a Web site dubbing itself “The Friend Society,” which links to various online conversations and off-color cartoons.

Authorities traced the site’s Internet provider back to Voxel Dot Net Inc., which has support and engineering staff based in Troy, N.Y. A man who answered the phone at Voxel, who declined to give his name, said he was unaware of the posted threat on the Web site and refused further comment.

The author of the threats, posted at 9:31 p.m. EDT on Oct. 12, identified himself online as “javness.”

“In the aftermath civil wars will erupt across the world, both in the Middle East and within the United States,” javness wrote. “Global economies will screech to a halt. General chaos will rule.”

Fellow online posters sounded skeptical about the claims.

“This isn’t something you should joke about,” a poster identified as “Kim Possible” wrote in response almost two hours later. “If you are (serious) about this may I see your sources. Unless you’re a psychic.”

Associated Press writers Dave Goldberg and Ralph Russo in New York, Kristie Rieken in Houston, Josh Dubow in Alameda, Calif., and Michael Marot in Indianapolis contributed to this report.