Terrorists may be conducting "dry runs" at US airports to test security before a possible attack, according to a Transportation Security Administration warning to airport screeners.
In at least four incidents over the past year, security screeners have found items in carry-on luggage - blocks of cheese taped to electrical components, for example - that resembled homemade bombs, according to the TSA's July 20 memo.
"Past terrorist attacks and plots show that such testing generally indicates attacks will soon follow," said the internal memo, which was posted on the Internet by NBC News on Tuesday.
None of the passengers in question has been linked with criminal or terrorist organisations so far, the memo said.
The United States has tightened airport security since the Sept. 11 suicide attacks by al Qaeda militants in 2001 using hijacked passenger planes. Passengers are now commonly required to remove shoes and belts and may not carry more than a small amount of toiletries or cosmetics on board.
The TSA described its unclassified warning as one of 90 released this year and said it had no specific information of a pending attack.
"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 a statement.
