I Am an Employer: How Do I… Complete Form I-9, Employment Eligibility Verification? [open pdf - 1MB]
"U.S. employers are required by law to verify the employment authorization of all workers they hire on or after November 6, 1986, for employment in the United States, regardless of the workers' immigration status. Employers who hire or continue to employ individuals knowing that they are not authorized to be employed in the United States, or who fail to comply with employment authorization verification requirements, may face civil and, in some cases, criminal penalties. Form I-9, Employment Eligibility Verification, must be completed for each newly hired employee, including U.S. citizens, permanent residents, and temporary foreign workers, to demonstrate the employer's compliance with the law and the employee's work authorization. Through the Form I-9 verification process, employers ensure that employees possess proper authorization to work in the United States and that hiring practices do not unlawfully discriminate based on immigration status."
U.S. Citizenship and Immigration Services: http://www.uscis.gov/