This Army Corps was under direct orders of Lieut. Gen. Ulysses S. Grant until May 24, 1864, when it was assigned to the Army of the Potomac. See: Official Records, Series I, Volume XXXVI, Part 1, page 113 (note at the bottom of the page).
Return of Casualties in the Union forces, Battle of North Anna, Pamunkey, and Totopotomoy, May 22-June 1, 1864 (Total Army of the Potomac): Official Records, Series I, Volume XXXVI, Part 1, page 164.