If you do not live along the border where there are U.S. Border Patrol stations, contact the Immigration a Customs Enforcement (ICE) division of the Department of Homeland Security (DHS). Dial 866- DHS-2ICE (866-347-2423) to "report suspicious activity".
Either make a report on this national hot line or ask for the number of the nearest ICE field office with an investigations division "duty agent."
In order to gain credibility with any government official, make a point of taking down the agent''s name and badge number. Be sure to be polite, but firm.
At the time, ask for the procedure for follow-up and call the agent back after a reasonable amount of time, to check on the progress of the investigation.