31 C.F.R. § 34.101
The Secretary of the Treasury will invest such amounts in the Trust Fund that are not, in the judgment of the Secretary, required to meet needs for current withdrawals. The Secretary may invest in interest-bearing obligations of the United States, having maturities suitable to the needs of the Trust Fund as determined by the Secretary. These obligations will bear interest at rates described in 31 U.S.C. 9702 , unless the Secretary determines that such rates are unavailable for obligations with suitable maturities. In that event, the Secretary will select obligations of the United States bearing interest at rates determined by the Secretary, taking into consideration current market yields on outstanding marketable obligations of the United States of comparable maturities.
31 C.F.R. §34.101