One of the most insidious tactics at a terrorist organization's disposal is the implementation of a sleeper cell. This consists of secret agents who receive specialized training in their home countries and are then assigned to assimilate into another country's culture and society. These agents may spend years performing their regular duties while living deep undercover, then suddenly receive orders from their overseas handlers to either commit an act of terrorism or provide aid to those who will.
Individual members of a sleeper cell may not even be aware of each other, since plausible deniability during police interrogations can be vital. One sleeper agent may work for an airline ticket office, for example, while another may work at a car rental company or a chemical plant. When the commanders of the terrorist organization want to activate a cell, each agent may only receive the name of one contact person or receive only his or her specific orders. The airline ticket agent, for example, may only be told to provide tickets for four men traveling from Germany to New York. The car rental agent may only be told to pick up these men from the airport and deliver them to the chemical plant. This process insures that no individual is aware of the entire plan.
It can be extremely difficult for government agencies to track and dismantle sleeper cells because of their nebulous construction. If the individual members are well-trained and dedicated to their cause, they can easily blend into society without raising any suspicions. A cell doesn't necessarily need to hold regular meetings or undergo additional training to carry out their missions. Members simply go about their daily routines until a handler contacts them for an assignment. Some agents may not even be fully aware of their obligations, believing that a benevolent government agency simply paid for their education and immigration expenses.
The events surrounding September 11 pointed out the dangers of such cells, since a number of US citizens did provide material assistance to the actual terrorists. While some may have simply been performing their regular duties, others were most likely members of sleeper cells activated to perform crucial support operations. A student pilot who only studied take-off procedures, for example, would be an ideal member. An airline baggage handler who could smuggle weapons onto a plane without notice would also be useful.
Some agents may be fully aware of their roles in an operation, while others may be deliberately kept in the dark until called upon to act. This is why an attack can be so notoriously difficult to thwart. A cell is often discovered through covert surveillance and infiltration by a double agent.