From Deutsche Bank:
The ownership structure of US Treasuries is fascinating: 50% is held by foreigners and 20% is held by the Fed. Domestic accounts hold only 30% of Treasuries, down from 75% in the early 1990s, see chart below.
Also, attached please find our 105 page update on Who is buying Treasuries, Mortgages, Credit, and Munis. Let us know if you want to set up a conference call to discuss flows with us and our trading desk.
Source: Deutsche Bank, Who is buying Treasuries, Mortgages, Credit, and Munis?