On the one hand the law requires that an employer verify the employment eligibility of every employee hired after the passage of the Immigration Reform and Control Act of 1986.  On the other hand it is illegal to discriminate in employment based on alienage or national origin.

Your I-9 Obligation.  You are required to complete the I-9 form part 1 on the date employment begins, part 2 no later than 3 days after the employee begins work.  In completing the form you will be required to ask for documents that demonstrate that the employee is authorized to work in the U.S.  This is so complex that the USCIS has published a 64 page manual with instructions on how to properly complete form I-9.  In one recent case ICE sought nearly $7,000 in penalties against a mom and pop Chinese restaurant because they had improperly completed just seven I-9 forms.