If the FBI had really done it, I doubt they would have left classified records saying they had. Most likely, either the records would have been destroyed long ago, or they never would have created any in the first place
Other possibility: they really didn’t do it, but there’s some classified record which sure makes it sound like they did. e.g. some record of J. Edgar Hoover joking about doing it
I think there’s a big difference between “random FBI field agent decides to murder someone” and “FBI director asks his deputy to murder someone”. Attributing the murder to the FBI as an institution makes a lot more sense in the second case than in the first
