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 threat claims that dirty bombs could be used this Sunday against seven stadiums -- in Miami, New York, Atlanta, Seattle, Houston, Oakland and Cleveland.

Read full report.