[Excerpt] Retirement annuities for civilian federal employees are provided mainly through two programs: the Civil Service Retirement System (CSRS) and the Federal Employees’ Retirement System (FERS). Under both of these systems, retirement annuities are financed through a combination of employee and employer contributions to the Civil Service Retirement and Disability Fund (CSRDF). All assets of the CSRDF are invested in U.S. Treasury bonds and other securities backed by the full faith and credit of the U.S. government. Retirement annuities for federal employees are paid from the CSRDF regardless of whether the benefits were accrued under CSRS or FERS. FERS annuities are fully funded by the sum of employee and employer contributions and interest earned by the Treasury bonds held by the CSRDF. The federal government makes supplemental payments into the CSRDF on behalf of employees covered by the CSRS because employee and agency contributions and interest earnings do not meet the full cost of the benefits earned by employees covered by that system.