Social Security Trust Fund

Total Holdings of US Treasuries: $2.67 trillion

Percent of US Debt that they own: 19%

The Old-Age and Survivors Insurance (OASI) and Disability Insurance (DI) Trust Funds invest exclusively in special issue bonds that are only available to the Social Security trust fund.  These are not publicly traded securities, but they still constitute a huge amount of debt.

Source: Social Security Administration