Foreign Policy has a fascinating piece up by J.M. Buerger detailing the FBI's years-long infiltration of the "Patriots Movement" more than two decades ago:
Starting in April 1991, three FBI agents posed as members of an invented racist militia group called the Veterans Aryan Movement. According to their cover story, VAM members robbed armored cars, using the proceeds to buy weapons and support racist extremism. The lead agent was a Vietnam veteran with a background in narcotics, using the alias Dave Rossi.
Code-named PATCON, for "Patriot-conspiracy," the investigation would last more than two years, crossing state and organizational lines in search of intelligence on the so-called Patriot movement, the label applied to a wildly diverse collection of racist, ultra-libertarian, right-wing and/or pro-gun activists and extremists who, over the years, have found common cause in their suspicion and fear of the federal government.
The undercover agents met some of the most infamous names in the movement, but their work never led to a single arrest. When [Oklahoma City bomber Timothy] McVeigh walked through the middle of the investigation in 1993, he went unnoticed.
The only criminal charges filed in relation to the "Patriot-conspiracy" investigation, in fact, were for the theft of Army night vision goggles, a crime that had been initially uncovered during the FBI investigation but was subsequently pursued by U.S. Army investigators separately.
Buerger uncovered many of the details about the FBI's infiltration of the Patriot Movement through FOIA requests and the whole thing is worth a read, especially given today's similar attempts to infiltrate entire Muslim communities. And for some historical context, here's a Reason review from 1996 of two books that came out on the militia movement in the aftermath of Oklahoma City.