Second, establish policies and procedures that help ensure better future compliance. Implement a comprehensive training program. For contractors that hire on the jobsite, it is especially important that hiring supervisors and managers understand how to properly complete I-9 forms. Consider enrolling in E-Verify if you're not already obligated to do so. If you are ever audited, this can be the best defensive tool at your disposal.
Third, ensure that your subcontractors and other vendors are in compliance also. Often, if ICE discovers a problem with one construction contractor, the investigation will expand outward to cover other contractors with whom the target is doing business. As a result, it is prudent to insist that your subs are in compliance and remain so. It may very well be worth it to provide training to your subcontractors and other vendors.
Finally, document your efforts to maintain good compliance. If ICE shows up, show them that you care and drown them in documentation. If you need help improving I-9 compliance, seek competent counsel rather than contacting the government directly.