On Tuesday, Homeland Security Secretary Michael Chertoff told the editorial board of The Chicago Tribune that he had a "gut feeling" about a new period of increased risk.
He based his assessment on earlier patterns of terrorists in Europe and intelligence he would not disclose.
"Summertime seems to be appealing to them," Chertoff said in his discussion with the newspaper about terrorists. "We worry that they are rebuilding their activities."
Other U.S. counterterrorism officials, who spoke on condition of anonymity, shared Chertoff's concern and said that al-Qaida and like-minded groups have been able to plot and train more freely in the tribal areas along the Afghan-Pakistani border in recent months. Osama bin Laden and his top deputy, Ayman al-Zawahri, are believed to be hiding in the rugged region.
"The threat coming out of there is very real, even if there aren't a lot of specifics attached to it," one of the officials said.
Chertoff's department has not made any move to increase the nation's color-coded terror alert system. Now, airlines are under orange — or high — alert, which is the second most serious level on a five-point scale. The rest of the country remains a step below at yellow, or elevated.