Generally, when a retired employee returns to work for the Federal Government under conditions that do not terminate the retirement benefit, the employee should be covered under the same retirement plan he or she had at retirement. Since you retired under CSRS, you should have retained your CSRS coverage upon reemployment. (However, if you received an appointment as a Senior Official, you would be subject to automatic Social Security coverage and your retirement coverage would be CSRS Offset.)
A CSRS or CSRS Offset retiree who is reemployed with the Federal Government may elect to join FERS within 6 months of the reemployment if the time between retirement and reemployment is more than 4 days and if the reemployment is under an appointment that is not excluded from FERS.
There is a special rule relating to retirement deductions for CSRS and CSRS Offset retirees who return to work under conditions that do not terminate the annuity. Although such an individual would be covered under CSRS or CSRS Offset, retirement deductions would not be withheld from the individual's salary unless the individual elected to have the deductions withheld.