President Bush has invoked executive privilege to block the testimony of two former aides and deny Congress key documents in its probe into the firing of nine U.S. attorneys. Monday morning was the deadline for the White House to explain its refusal to comply. In a letter to the House and Senate Judiciary Committees, White House Counsel Fred Fielding said former Karl Rove aide Sara Taylor and former White House counsel Harriet Miers were covered under Bush’s executive privilege.