40 ILCS 5/21-107
Any position in the service of a State agency, department, board or commission which (1) has been designated as a fireman, police officer, or law enforcement officer by the State Department of Central Management Services, the State Attorney General or the State Agency, or declared to be a fireman or police position as a result of a court ruling upon the question, and (2) extends to the incumbent the right to participation in the State Employees' Retirement System, shall be exempted from Social Security coverage in compliance with Section 218(d)(5)(A) of the Social Security Act, effective as of the date of such designation or court ruling, except that beginning July 1, 1991, incumbents of such positions who do not participate in the State Employees' Retirement System shall be subject to mandatory Social Security coverage.
40 ILCS 5/21-107