Neb. Rev. Stat. § 29-1902

If the subpoena be served by such special deputy, it shall be his duty, after serving the same, to return thereon the manner in which the same was served; and also to make oath or affirmation to the truth of such return, before some person competent to administer oaths, which shall be endorsed on such writ; and the same shall be returned according to the command thereof by the person serving the same through the post office or otherwise.