"The Social Security trust funds, managed by the Department of the Treasury, are the Old-Age and Survivors Insurance (OASI) and Disability Insurance (DI) Trust Funds. Since the beginning of the Social Security program, all securities held by the trust funds have been issued by the Federal Government.
There are two general types of such securities:
special issues—securities available only to the trust funds; and
public issues—securities available to the public (marketable securities).
The trust funds now hold only special issues, but they have held public issues in the past."
(http://www.ssa.gov/oact/progdata/investheld.html)