FBI infiltration of US mosques

I was in San Bernardino and a Muslim student picked me up from my hotel to take me to the university campus.  He is active in the local Muslim community--he told me--and even knew the San Bernardino shooter (unsurprisingly he told me that he was a very nice guy and he does not believe the official government story about what happened).  So I asked him: how obvious  are FBI infiltrators in the mosques in the US?  He said: way too obvious.  He explains that the community of the mosque all know one another and they know each other's families.  And then suddenly, this white dude (or a Lebanese Christian dude) shows up in the mosque and makes an effort to know the members.  He said that it is so stupid that the informant would invariably ask members: so, tell me what do you think of Jihad?