The Federal Reserve’s relentless acquisition of agency MBS has been the biggest factor in the changing complexion of the MBS investor picture, far outstripping the tortoise-like pace at which the market has expanded this year. The Fed increased its agency MBS holdings by 12.8 percent during the second quarter of 2013, according to a new Inside MBS & ABS profile of investor classes in the mortgage securities market. The central bank added $137.2 billion to its agency MBS holdings during the quarter, and its gross acquisitions totaled $466.6 billion since the beginning of the year. That represented...[Includes two data charts]