A short article by Peter Bergen, to accompany a HBO documentary, 'Homegrown: the CT dilemma' broadcast last Monday, so behind a paywall:
A very telling indicator of future violence by a terrorist, FBI behavioral analysts have found, is what they term "leakage."..Leakage is, in short, when a violent perpetrator signals to people in his circle that he is planning an act of violence...In an ongoing study of some 80 terrorism cases in the States since 2009, which has not been previously reported, the FBI found that "leakage" happened more than 80% of the time...Strangers were the most likely to come forward.. rather than the 95% of the peers, family members and authority figures who generally had the most useful information about a militant.
(He concludes) The lesson of the FBI study of terrorism cases is that the most useful information comes from peers and family members. That's why community outreach to Muslim communities to enlist their help in detecting those who may be becoming militant is the most fruitful approach to dealing with the scourge of terrorism.
Link:http://edition.cnn.com/2016/02/03/op...eakage-bergen/

He does not cite the actual FBI report and a look on Google failed.