Sleeper cells are a real 'potential' terrorist phenomenon as discussed by the FBI and US Senate House subcomittees mentioned below. Sleeper cells are different from clandestine cell structure terrorist networks in that they lie dormant until activated by a prearranged or acceptable sudden signal to perform acts of espionage, sabotage and/or terrorism.
Michael Cutler's prepared testimony before House Subcommittee on Immigration, Border Security and Claims on the topic, "Interior Immigration Enforcement Resources." which is present here.
FBI 2005 secret report.
Referring to ABC news, a secret FBI assessment in 2005 concludes that, "To date, we have not identified any true 'sleeper' agents in the US." The report however does cite several cases of potential sleeper agents which included a member of the Saudi Arabian Air Force training at Lackland Air Force Base in Texas.
"Individual operatives who possess a clean passport, have not come to the attention of intelligence agencies overseas, and lack a criminal record are unlikely to attract the attention of security agencies in the United States, unless they are in contact with known extremists," according to the report. "Al-Qa'ida has altered its operative profile, making it more difficult to screen visa applicants at embassies and individuals entering the United States at airports and other border crossings."
There is little to no evidence of any specific sleeper cells in several countries of the world but John Guandolo, former FBI special agent and counterterrorism expert tells that the goal of extremist groups is to have camps established in the United States and sleeper cells ready to act when called upon.
By early 2005 there was little to no evidence of Al Qaeda sleeper cells in the US. When he testified before the Senate Select Committee on Intelligence in February 2005, FBI Director Robert S. Mueller III testified that he is "very concerned" about the lack of data on a network of al Qaeda "sleeper" cells in the United States. He went on to say that, "Finding them is a top priority for the FBI, but it is also one of the most difficult challenges." In March 2005 a leaked FBI report concluded that "US Government efforts to date also have not revealed evidence of concealed cells or networks acting in the homeland as sleepers."
- Referring to French Interior Minister Bernard Cazeneuve's interview, European intelligence services which includes countries like France are tracking hundreds of people believed to belonging to possible sleeper cells for terror organizations.
“Four hundred targets have been identified by our intelligence services that are more or less sleeper cells, affiliated or in relation with al Qaeda-type organizations, that can strike like the Kouachi brothers,” Cazeneuve said in an interview late Monday. “When you have terrorists who keep a low profile for years and then suddenly decide to act — either to obey an order from large terrorist organization such as al Qaeda or of their own volition — then you need to be able to monitor them on a long term,” the minister said.
Referring to a study by Daniel B. Kennedy and colleagues into sleeper cells in 2008, Pipes' 2003 checklist of indicators of possible sleeper cell terrorists appeared to be fairly comprehensive in determining sleeper cell profile items.
During the course of our research we also confirmed the existence of at least three basic perspectives on the question of sleeper cells, as represented by the critic, defender, and mainstream subgroups. Further research will be necessary to identify the proportions of these and other subgroups. Our research raises the issue of whether a terrorist profile can be constructed independently of the perspective of the person employing it.