That ex undercover chap did at least point out that it’s not necessarily the cause that concerns law enforcement agencies, but the tactics used and crimes committed supposedly in the name of that cause.
That's very important. The FBI under Hoover was very effective against extremism - we hear about their actions against the left, but they also completely crushed the Klan. They did, it's true, go way overboard with the Cointelpro strategy of infiltrating and disrupting groups who might have had extremist ideologies but who had entirely peaceful methods.
That's been the basis for a lot of the mythology that's grown up on the American left, and not only the far left but the centre left, who will wang on endlessly about the Hollywood blacklist while airbrushing the fact that there was significant violence from far left groups, that Communist Party members were heavily involved in espionage etc.
Kash Patel will be worth watching because he's argued for years that the FBI was far too heavily involved in politics and should really get out of the business of surveilling activism and back into the business of solving crime. I don't know how he'll jump.
It's even trickier if we're talking about decentralised and networked extremism. And it might only be a small number of violent extremists, but they're swimming in the sea of a community of people with lots of mental health issues, where violent rhetoric has become completely normalised.