Bingo! Durham is investigating the FBI why would be employ them to investigate anything. Respectable sources I follow on Twitter indicate Durham has historically used Postal Inspectors or DOJ IG investigators. This raid appears to be an attempt to either muddy the water for Durham or serve as a not so subtle shot across the bow.
The swamp is the swamp.
The FBI could be after Deripaska for a range of actual crimes, but I suspect the purpose is to prevent him from cooperating with Durham and/or seize any evidence that would help Durham’s investigation (such as evidence of communications with Alan Waldman and Mark Warner) thereby deep sixing it forever.