In the late 1930s, the FBI began infiltrating the commies. It was joked that the Communist Party in the US would go broke if it weren't for the FBI dues. A very effective attack.
The approach carried over very nicely to the evil militias in the 60s, 70s and 80s. Every militia unit in the country was well infiltrated by FBI agents and/or sources. Thsu, the FBI knew exactly what Nichols and crew were planning. In fact, the FBI had a trap for them on the morning of the blast. Local police were called out and they were waiting. But, the FBI big shots got hung up and didn't get to OKC in time and NO ONE had the brass to act independently and the blast went, dentonating material improperly stored in the Murrah Building.
Each of the militia unit was protectng the others including the FBI contacts (who were not known to be spies). That is why the one militiaman was rushed to his execution.
No, we will never have the proof of the FBI involvement for many, many years.
The quote from Michael Rivero in the original post says it all.