"...the fourth amendment as construed in
Terry requires that the police officer have an articulable reasonable su ion of crime to make the initial stop but the new arizona law adds an additional twist: if, after the initial stop, the officer develops a further reasonable su ion that the detainee is an undo ented immigrant, the officer then must take steps to ascertain the detainee's immigration status.
Hunches are only as good as the base of experience on which they rest. A person who has had numerous opportunities to sort undo ented immigrants from persons lawfully present in the United States–such as a federal Immigration and Customs Enforcement (ICE) agent–might develop an intuitive ability or "sixth sense" for detecting the former. But even then we would legitimately worry that skin color and accent would often play a large part in triggering the ICE agent's hunch. Moreover, Arizona state and local officials who lack training and experience sorting out undo ented immigrants would likely lean even more heavily on such illicit factors.
To be sure, there will be some cir stances that objectively give rise to reasonable su ion, even without considering race or ethnicity.
Writing in the New York Times last week, one of the law's authors gave the example of a police officer stopping a speeding minivan on "a known alien-smuggling corridor" and discovering twelve "passengers crammed inside," all lacking identification. Surely that would give rise to reasonable su ion of immigration violations, he says. Indeed, that extreme case would satisfy the more demanding requirements of probable cause. But the Arizona law applies in numerous other contexts as well.
The critics of the new Arizona law are right. The core problem with the obligation to investigate that it places on state and local officials is that, in most settings, there are very few if any outwardly visible signs of immigration status that could give rise to reasonable su ion.
That will be true regardless of whether the officials act on expressly-articulated grounds or on the basis of a hunch informed by unarticulated grounds. Thus, whatever one thinks of how "reasonable su ion" has been defined in the federal Fourth Amendment context, its use in the Arizona law is problematic."
http://writ.news.findlaw.com/dorf/20100503.html