This is copied from a post added 23/1/10 to a thread on Turkey's experience.

One of the few open source articles that provides some context for the role of the public is from Turkey:http://ccj.sagepub.com/cgi/content/abstract/23/2/142 (Behind a pay wall, although I have a copy).

In November 2003, a series of coordinated suicide bombings were carried out by al-Qaeda in Istanbul. The targets represented Israel and the West, including two synagogues, an HSBC bank, and the British consulate. The attacks resulted in 68 deaths and more than 700 injured. The investigation and arrests that ensued revealed that the network involved in the bombings had trained in Afghanistan. Of particular interest was the interpersonal web that grew from the four suicide bombers as well as the range of materials confiscated in the investigation. (My emphasis)Specifically, nearly 300 people were identified who had some knowledge of the planned attack. Of these, 48 were viewed as hard-core committed terrorists, leaving approximately 250 community members who were not ideologically committed to al-Qaeda’s goals and who had some information that potentially could have been used in preventive action.
Chilling and possibly a reason why it is the only example in the public domain I know of, with numbers.