The FBI in 2008 estimated it had 15,000 paid informants. About 30% of post-9/11 terrorism cases are considered sting operations in which informants played an “active role” in incubating plots leading to arrest, according to studies cited in the Human Rights Watch report.

http://www.theguardian.com/world/2014/jul/21/government-agents-directly-involved-us-terror-plots-report