In 2007 Ed Morrisey summarized a series of apparent terrorist dry-runs. At that time the TSA alert (PDF) suggested that an attack on airliners was imminent. The TSA noted items that closely resembled C4 (explosives), IEDs, and remote triggers.
Furthermore, the explanations of the persons involved were "questionable" and investigations continue.
[Authorities] have intercepted several items at airports resembling improvised explosive device (IED) components. These items include wires, switches, pipes or tubes, cell phone components, and dense clay-like substances. The unusual nature and increase in number of these improvised items raise concern... Terrorists may repeat operational tests to desensitize, distract, or adapt plans for specific environments...
Individuals involved in these incidents were of varying gender, and initial investigations do not link them with criminal or terrorist organizations. However, most passengers’ explanations for carrying the suspicious items were questionable, and some investigations are still ongoing. Many of the incidents caused airport terminal evacuations and at times, brief closures...
Noteworthy incidents include the following:
5 July 2007, San Diego, CA – A U.S. Person’s (USPER) checked baggage contained two icepacks covered in duct tape. The icepacks had clay inside them rather than the normal blue gel.
4 June 2007, Milwaukee, WI – The carry-on baggage of a USPER contained several 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
8 November 2006, Houston, TX – A USPER’s checked baggage contained a plastic bag with a 9-volt battery, wires, a block of brown clay-like minerals, and pipes.
16 September 2006, Baltimore, MD – The checked baggage of a couple contained a plastic bag with a block of processed cheese taped to another plastic bag holding a cellular phone charger.
At some point, the TSA practices -- often described as "security theater" -- need to cease. They must be replaced with decisive, intelligent measures such as those used by Israeli security personnel.