WASHINGTON (CNN) -- Police across the country should be on the lookout for what could be "dry runs" for a terrorist attack, the Transportation Security Administration advised after series of suspicious incidents occurred at U.S. airports.
Blocks of cheese were found in a bag with a coil of wire and tubes, the TSA said.
An unclassified advisory, sent July 20 from TSA to law enforcement agencies, raised the possibility that recent activity could be "pre-attack security probes."
CNN obtained the advisory from a government source.
The TSA downplayed the significance of the advisory in a statement released to the media following its leak.
The TSA said it was one of more than 90 bulletins sent to police in the past six months "with the intent to provide as much information as possible to our front line officers."
"There is no intelligence that indicates a specific or credible threat to the homeland," the TSA said.
The advisory details four incidents from the past 11 months in which screeners found unusual objects with items that could mimic bomb components in passengers' checked or carry-on bags. Watch how cheese was made to look like a bomb »
In one case last September, a couple in Baltimore, Maryland, checked a plastic bag with a block of processed cheese taped to another plastic bag containing a cell phone charger. Earlier this month in San Diego, California, a passenger checked a bag containing two ice packs covered in duct tape. The ice packs had clay in them instead of the normal blue gel.
Terrorists could be testing the system, or could be conducting repeated operations to desensitize security officials, the bulletin says.
"The unusual nature and increase in number of these improvised items raise concern, and TSA personnel should continue vigilance for groupings of ordinary items that look like IED (Improvised Explosive Device) components," the bulletin says.
At least three of the four incidents involved U.S. citizens, the advisory says. While "initial investigations do not link them with criminal or terrorist organizations," the bulletin adds that "most passengers' explanations for carrying the suspicious items were questionable, and some investigations are still ongoing."
The bulletin also details a case from June in which a passenger in Milwaukee, Wisconsin, had a carry-on bag with items resembling IED components, such as a wire coil wrapped around a possible initiator, an electrical switch, batteries, three tubes and two blocks of cheese.
And in November, a passenger in Houston, Texas, checked luggage that contained a plastic bag with a 9-volt battery, wires, a block of brown clay-like minerals and pipes.
The bulletin is titled "Incidents at U.S. Airports May Suggest Possible Pre-Attack Probing." It is labeled "For Official Use Only."
"We constantly feed intelligence and training information to our officers and the law enforcement community and this is one example of such information sharing," the TSA said in its statement. E-mail to a friend
CNN's Kevin Bohn contributed to this report.
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