“Mr. McCabe said in an interview that had he known Mr. Mueller would not continue the inquiry, he would have had the F.B.I. perform it.” https://www.nytimes.com/2020/08/30/us/politics/trump-russia-justice-department.html

A lot of people shared McCabe’s assumption, I think, because the Mueller investigation was nearly always described as continuing the investigation that Comey had disclosed at the infamous 2017 HPSCI hearing—which he had said was part of the FBI’s counterintelligence mission. https://twitter.com/ivanthek/status/1300092314866659330

Here’s what Comey said at the hearing and Rosenstein’s public order appointing Mueller, which scopes the investigation based on that testimony.

It was not public knowledge that Rosenstein privately excluded counterintelligence from the investigation.

I’ve had so many conversations where someone has said some version of “Mueller must have looked at Trump’s tax returns.” But it’s not the case. All the evidence we have says he didn’t.

It’s not clear that any law enforcement entity has ever examined his tax returns in detail.