In a new intelligence bulletin obtained by NBC News, the Transportation Security Administration is warning law-enforcement and airport officials that terrorists "may be" conducting "preattack security probes" at U.S. airports.

The unclassified bulletin, dated July 20 and marked "For Official Use Only," is titled "Incidents at U.S. Airports May Suggest Possible Pre-Attack Probing."

"A surge in recent suspicious incidents at U.S. airports may indicate terrorists are conducting pre-attack security probes and 'dry runs' similar to dress rehearsals," the intelligence bulletin states. "Past terrorist attacks and plots show that such testing generally indicates attacks will soon follow, according to a joint FBI and Homeland Security assessment."

he report says that TSA personnel have intercepted items at U.S. airports "resembling improvised explosive device (IED) components," including wires, switches, pipes and "dense clay-like substances" from September 2006 to date.

The items were confiscated at airports in San Diego, Milwaukee, Houston and Baltimore. In two of the four incidents, blocks of processed cheese were connected to wires and an electrical switch, or the cheese was taped to a bag holding a cell-phone charger.

A U.S. law enforcement official cautions NBC News that the new bulletin is an analysis of possible suspicious behavior at U.S. airports and not at all a sure sign of terrorists at work. The bulletin is just one of a stream of intelligence reports constantly churned out by TSA, the Department of Homeland Security, the FBI and other U.S. agencies.

The official also warns that the items seized in recent months at U.S. airports may indicate a warning sign but also may be completely innocent and coincidental.

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