Who said anything about a “secret” investigation? You’re making that up. The FBI lawyers confirmed it as an ongoing investigation. I will post the excerpt of the article again (emphasis mine):
...the FBI general counsel present for the interview would not allow Baker to answer a series of questions about topics like surveillance of Trump campaign aides because it is tied to ongoing investigations.
So, there is an ongoing investigation into the “surveillance of Trump campaign aides”.
My question is, who is doing this “ongoing” investigation? You said it was Mueller.
Descriptions of testimony are hearsay, as it was a closed meeting. Parsing phrases imagined by a reporter is pointless.
The only confirmed investigation into this whole area is Mueller’s.
P.S.: The Mueller investigation is reported to be finished. So it can’t be the “ongoing” investigation referred to by the FBI lawyers in this article.
Read between the lines...