The unclassified alert was distributed on July 20 by the Transportation Security Administration to federal air marshals, its own transportation security officers and other law enforcement agencies.
CBS News correspondent Bob Orr says U.S. officials stress that this is a routine advisory – one of about 90 similar "bulletins" the TSA has circulated to its workers since January, as a reminder to baggage screeners, air marshals and other security officials to take a very broad "eyes open" approach to their jobs.
The bulletin, which does not constitute a heightened state of alert, tells TSA workers that items seized in separate incidents at the airports in San Diego, Milwaukee, Houston and Baltimore airports have included "wires, switches, pipes or tubes, cell phone components and dense clay-like substances," including block cheese. "The unusual nature and increase in number of these improvised items," the TSA says in its advisory, "raise concern."
Security officers are urged to keep an eye out for "ordinary items that look like improvised explosive device components" – that is, things that could be used to build a homemade bomb.
Word of the advisory surfaced on the same day as two other reports which could put American nerves on edge: comments byon the potential for al Qaeda terror cells within the fifty states, and a non-terror related warning from who say poor maintenance of airport facilities could endanger the flying public.
The TSA circulated its 13-paragraph security advisory internally and not to reporters – who started questioning Bush administration officials after the document leaked out to the media late Tuesday and was posted on the web.
"There is no credible, specific threat here," said TSA spokeswoman Ellen Howe. "Don't panic. We do these things all the time."
The Department of Homeland Security and the FBI both say that while aviation remains a prime terrorist target, they see no credible evidence of any imminent attack in the U.S.
The alert level for the nation's airports remains Orange, which means "High," while the alert level for the rest of the country is unchanged at Yellow, which means "Elevated."
Homeland Security spokesman Russ Knocke described the TSA advisory as the latest copy of a routine informational bulletin.
A statement posted late Tuesday by the TSA on its Web site confirms that "a routine TSA intelligence bulletin relating to suspicious incidents at U.S. airports" had leaked out to news organizations.
The bulletin says a joint FBI-Homeland Security Department assessment found that terrorists have conducted probes, dry runs and dress rehearsals in advance of previous attacks.
It cites various types of rehearsals conducted by terrorists before the Sept. 11, 2001, attacks on the World Trade Center and the Pentagon; the July 7, 2005, London subway bombings; the Aug. 2, 2006, London-based plot to blow up trans-Atlantic flights using liquid explosives and the 1994 Bojinka plot in the Philippines to blow up multiple airliners over the Pacific Ocean.
The bulletin says initial investigation has not linked the men and women carrying the suspicious items to any terrorist or criminal organizations. The TSA adds that most of the explanations given by passengers caught with subsequently seized items have been suspicious in themselves and some are still under investigation.
The four seizures are described as follows: