The threat from domestic terrorism is rising, but, with Republicans decrying the “deep state,” the F.B.I. is cautious about investigating far-right groups. Vigilantes are leaping into the fray.

I cannot speak directly to that, and likely as not we'll never know. However historically fascists often get the benefit of doubt from whichever conservatives are in power when the fascists are active. It's how fascists often rise to power.

I agree with your initial point, also I wasn't explicitly clear. Tbe FBI are the US Top Cops, and well, cops don't like to investigate other cops.

https://hrlr.law.columbia.edu/hrlr-online/when-police-mess-up-the-lack-of-a-defense-to-inadequate-police-investigations/