Airport security officers around the nation have been alerted by federal officials to look out for terrorists practicing to carry explosive components onto aircraft, based on four curious seizures at airports since last September.
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.
The seizures at airports in San Diego, Milwaukee, Houston and Baltimore included "wires, switches, pipes or tubes, cell phone components and dense clay-like substances," including block cheese, the bulletin said. "The unusual nature and increase in number of these improvised items raise concern."
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Security officers were urged to keep an eye out for "ordinary items that look like improvised explosive device components."
The 13-paragraph bulletin was posted on the Internet by NBC Nightly News, which first reported the story.
A federal official familiar with the document confirmed the authenticity of the NBC posting but declined to be identified by name because it has not been officially released.
"There is no credible, specific threat here," TSA spokeswoman Ellen Howe said Tuesday. "Don't panic. We do these things all the time."
Homeland Security spokesman Russ Knocke described the notice as the latest copy of a routine informational bulletin for TSA workers, airport employees and law enforcement officials.
A statement posted late Tuesday by the TSA on its Web site confirmed that "a routine TSA intelligence bulletin relating to suspicious incidents at U.S. airports" had leaked to news organizations. The statement added, "During the past six months TSA has produced more than 90 unclassified bulletins of this nature on a wide variety of security-related subjects."
The bulletin said the a joint FBI-Homeland Security Department assessment found that terrorists have conducted probes, dry runs and dress rehearsals in advance of previous attacks.
It cited 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 said the passengers carrying the suspicious items seized since September included men and women and that initial investigation had not linked them with criminal or terrorist organizations. But it added that most of their explanations for carrying the items were suspicious and some were still under investigation.
The four seizures were described this way:
— San Diego, July 7. A U.S. person — either a citizen or a foreigner legally here — checked baggage containing two ice packs covered in duct tape. The ice packs had clay inside them rather than the normal blue gel.
— Milwaukee, June 4. A U.S. person's carryon baggage contained wire coil wrapped around a possible initiator, an electrical switch, batteries, three tubes and two blocks of cheese. The bulletin said block cheese has a consistency similar to some explosives.
— Houston, Nov. 8, 2006. A U.S. person's checked baggage contained a plastic bag with a 9-volt battery, wires, a block of brown clay-like minerals and pipes.
— Baltimore, Sept. 16, 2006. A couple's checked baggage contained a plastic bag with a block of processed cheese taped to another plastic bag holding a cellular phone charger.
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