Neb. Rev. Stat. § 79-9,118

No employee shall be authorized to participate in the retirement system unless the employee is a United States citizen or is lawfully present in the United States. The employing public school and the school employee shall: (1) Maintain at least one approved identification document as defined in section 4-108 ; and (2) Produce any such approved identification document so maintained upon request of the Public Employees Retirement Board or the Nebraska Public Employees Retirement Systems.