One detail of the FBI's 2013 investigation into Omar Mateen that seems to be getting inadequate attention is that they used multiple informants with him, per Jim Comey's press conference on Monday:
Our investigation involved introducing confidential sources to him, recording conversations with him, following him, reviewing transactional records from his communications, and searching all government holdings for any possible connections, any possible derogatory information. We then interviewed him twice. [my emphasis]
Normally, when the FBI identifies a Muslim mouthing off about joining ISIS, they throw one or more informants at him, develop his trust, then have him press a button or buy a plane ticket to Syria, which they use to arrest the guy.
That didn't happen here. While they did record the conversations between these informants and Mateen, they never got him to do something they could arrest him for.
And I suspect we won't get answers why they didn't, though it seems an absolutely critical question for assessing how the FBI investigates terrorism. If FBI's chosen method of using informants only works with the dopes and not the real threats, all it does is juice the FBI's prosecution numbers, without keeping us safe. Alternately, it's possible FBI assumes certain things about a potential "Islamic" threat, which turned out to be wrong in this case.
I can think of several possible reasons why FBI's informants might not have worked the way they normally do (these are speculative):
- Mateen was just not serious about terrorism in 2013, but something since then (perhaps the decline in his marriage, perhaps the US launching yet another war against Muslims in the Middle East) led him to embrace it in 2016
- Mateen, who went to cop school, recognized the informants for what they were
- The prominent reporting on FBI's investigations into Ibragim Todashev and their infiltration of his circle of friends (the FBI's investigation would have lasted from July 2013 until May 2014) made Mateen vigilant enough to resist the informants' appeals
- The informants tried to entice Mateen via Islamic ideology and not homophobic self-hatred (that is, they used the wrong trigger)
- The process of being investigated -- and interviewed 3 times -- actually further pissed off Mateen, leading him closer to violence
Again, these are all speculative. We can't know without more detail why the FBI's typical use of informants failed this time.
But we deserve answers to the question, because if the Muslim community is going to be riddled with informants, they had better be serving some purpose other than selective surveillance of a minority group.