What is the test for reasonable suspicion of illegal immigration?

In light of the above principles, we conclude that the facts in this case "describe behavior that should excite the suspicion of a trained border patrol agent that criminal activity is afoot." United States v. Rodriguez, 976 F.2d 592, 594 (9th Cir.1992), amended, 997 F.2d 1306 (9th Cir.1993). The following facts gave rise to reasonable suspicion:

Agents spotted three men and six horses 20-30 feet from the border, "trying to hide" under some trees;

The riders had in tow three horses carrying empty sacks;