He's supposed to have been specifically working on this investigation with Horowitz. I'd like to see the proof of what he's been doing. He'd likely have to document how many hours he's spent on this, along with whatever court cases he's been involved in, if any. There should still be some accountability for how your time is spent on the job, no matter how high up the ladder you go.
He was ‘working on it’ in addition to his regular duties, which also include serving on the ‘Attorney General’s Advisory Committee’.
US Attorneys don’t get assigned to one case or group of cases. The DOJ establishes ‘task forces’ for special projects, and usually add staff from outside the DOJ.