Trevor Aaronson, Investigative Reporting Fellow at the University of California, Berkeley, discusses his article “The Informers” that looks at the FBI’s prosecution of terrorism cases in the US; the huge increase of government informants since 2004, and whether they are exposing terrorist plots or manufacturing them; why an “entrapment” legal defense simply doesn’t work, even when it really should; and several specific cases of informants-run-amok, from Lodi, California to Miami,...