A threat that has been talked about a lot recently is the concept of a terrorist sleeper cell: Terrorists leading normal lives integrated into society for years, sometimes decades, until one day, they get "activated", i.e. receive a call, letter, email etc. with instructions for executing some plot/attack. Not only have government agencies repeatedly made reference to such structures; they also feature prominently in most fiction about terrorism.
The Wikipedia article on the subject is quite lengthy, and includes a whole section on sleeper cells as (allegedly) employed by al-Qaeda, but not a single example of an actual (planned or executed) terrorist act by such a unit; nor have I been able to find any concrete such examples anywhere else.
My simple question is therefore for a reliable source* confirming that "terrorist sleeper cells" actually exist.
* Note that given government agencies' recent history of deliberate misinformation on that subject, it is safe to say that a mere claim by such an agency of the form "the threat is very real, we have evidence [but are not showing it to you]" is not a reliable source.