He's been working on the secret case since July 2018. Likely about Qatari investment Authority, purchase of 19.5% of Rosneft, in partnership with Glencore (Marc Rich), financed by VTB, laundered through Singapore.
They've been fighting the subpoena since then, and it was finally upheld by the supreme court on the day Mueller issued his final report.
I cant understand why more people are not digging on this. Its ground central to Russia and mentioned in the dossier, though attribute to Page and Trump instead of clearly tying back to the Clinton's.