The general rule is that local police are not precluded from enforcing federal statutes. Ker v. California, 374 U.S. 23, 83 S.Ct. 1623, 10 L.Ed.2d 726 (1963); Miller v. United States, 357 U.S. 301, 78 S.Ct. 1190, 2 L.Ed.2d 1332 (1958); Johnson v. United States, 333 U.S. 10, 68 S.Ct. 367, 92 L.Ed. 436 (1948); United States v. DiRe, 332 U.S. 581, 68 S.Ct. 222, 92 L.Ed. 210 (1948). Where state enforcement activities do not impair federal regulatory interests concurrent enforcement activity is authorized. Florida Avocado Growers v. Paul, 373 U.S. 132, 142, 83 S.Ct. 1210, 1217, 10 L.Ed.2d 248 (1963). Therefore, federal regulation of a particular field should not be presumed to preempt state enforcement activity "in the absence of persuasive reasons--either that the nature of the regulated subject matter permits no other conclusion, or that the Congress has unmistakably so ordained." De Canas v. Bica, 424 U.S. 351, 356, 96 S.Ct. 933, 936, 47 L.Ed.2d 43 (1976) (quoting Florida Avocado Growers, 373 U.S. at 142, 83 S.Ct. at 1217).
14
Although the regulation of immigration is unquestionably an exclusive federal power, it is clear that this power does not preempt every state activity affecting aliens. De Canas, 424 U.S. at 354-55, 96 S.Ct. at 935-36. The plaintiffs' reference to exclusive federal authority over immigration matters thus does not resolve this question. Instead, we must define precisely the challenged state enforcement activity to determine if "the nature of the regulated subject matter permits no other conclusion."