From the FRBNY:
Purchases of agency MBS by investment managers acting as agents for the System Open Market Account (SOMA).
(Click image for a sharper version)
Hope they kept the receipt!
And from Brad Setser at the CFR, this graph. Quite a lot of ground for the Fed to make up to restore normalcy to the agency market.
Related links:
Coping with failure, redux – FT Alphaville
The 4.375% Fixed-Rate Mortgage – Market Movers

