The FBI made the announcement after agents questioned a 20-year-old Milwaukee man in an effort to determine who made the threats, which were posted on a Web site last week.
"The investigation has determined that this is a hoax. The public should be reassured of their security as they continue to attend sporting events this weekend," said the agencies' joint statement.
"From the information we have, we believe he was involved to some extent, but we don't know at what level," said FBI agent Douglas E. Porrini. He added: "That person was released, but we're not saying that he won't be charged."
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 Usama bin Laden would claim responsibility.
The Homeland Security Department alerted authorities Wednesday in the cities mentioned, as well as the NFL and the National Collegiate Athletic Association. But the FBI and Homeland Security said there was no intelligence indicating such an attack might be imminent.
"I don't think it was put out there to be real," said FBI agent Linda Krieg in Milwaukee. "Whoever put it out there is not in a position to actually carry through on it. It was not a credible threat."
The man being questioned did not appear to have any ties to terrorist groups, according to an FBI official in Washington who spoke on condition of anonymity because the investigation was still going on.
NFL spokesman Greg Aiello said stadiums are well protected through "comprehensive security procedures" that include bag searches and pat-downs.