Wait you mean you are mandated to do that by federal law?
Title 8 U.S.C. § 1324a(a)(1)(A) makes it unlawful for any person or other entity to hire, recruit, or refer for a fee, for employment in the United States an alien knowing the alien is an unauthorized alien, as defined in subsection 1324a(h)(3).
Subsection 1324a(2) makes it unlawful for any person or entity, after hiring an alien for employment, to continue to employ the alien in the United States knowing the alien is or has become an unauthorized alien with respect to such employment.
Subsection 1324a(f) provides that any person or entity that engages in a "pattern or practice" of violations of subsection (a)(1)(A) or (a)(2) shall be fined not more than $3000 for each unauthorized alien with respect to whom such a violation occurs, imprisoned for not more than six months for the entire pattern or practice, or both. The legislative history indicates that "a pattern or practice" of violations is to be given a commonsense rather than overly technical meaning, and must evidence regular, repeated and intentional activities, but does not include isolated, sporadic or accidental acts. H.R.Rep. No. 99-682, Part 3, 99th Cong., 2d Sess. (1986), p. 59. See 8 C.F.R. § 274a.1(k).A scheme for civil enforcement of the requirements of § 1324a through injunctions and monetary penalties is set forth in § 1324a(e) and § 1324a(f)(2).
In addition, 18 U.S.C. § 1546(b) makes it a felony offense to use a false identification document, or misuse a real one, for the purpose of satisfying the employment verification provisions in 8 U.S.C. § 1324a(b).