Who would send plainclothes officers to conduct riot control, especially ones who had no training in riot control?
That gob stinks on ice.
You want riot control police to be easily identifiable to the crowd, and especially each other. Khakis and t-shirts just don't do it in a scrum on the street, and you can't tell me they didn't have jackets and shirts and vests (body armor) that identified them as FBI.
The whole purpose of plainclothes duty is to blend in, to infiltrate, and in the case of the FBI (which has been notorious for this) to get people fired up to commit crimes they might have only mentioned in passing.
We formed a group during the Clinton era to badger legislators by mail and phone messages, in opposition to any new (and some existing) gun control measures.
We had a couple of folks pop in during meetings and try to divert our basic mission, and they were shut down promptly. (They did not come back).
They were told that what they proposed had nothing to do with our one-issue mission, and while they were free to discuss anything they wanted with folks outside the meeting, that just was not what we were there for.
The Guy in the room who is most shrill about burning things down or blowing them up is most likely a Fed. (See also: on video, Ray Epps, calling for going into the Capitol--before Trump's speech--and the people around him saying "Fed..Fed...Fed...")