Here's a fascinating new decision on how the Fourth Amendment regulates thermal imaging devices: United States v. Kattaria. In this case, the Eighth Circuit (Loken, joined by Wollman and Gibson) rules that the police only need reasonable suspicion to obtain a "warrant" to conduct thermal imaging monitoring of a home. The Court's rationale is based on a policy argument:

If the same probable cause is required to obtain both [warrants to search homes and warrants to use imaging devices], law enforcement will have little incentive to incur the expense of a minimally intrusive thermal imaging search before conducting a highly intrusive physical search. For these reasons, we are inclined to believe that the same Fourth Amendment reasonable suspicion standard that applies to Terry investigative stops should apply to the issuance of a purely investigative warrant to conduct a limited thermal imaging search from well outside the home.

  This is a strong policy argument but a weak legal one. In Kyllo v. United States, the Supreme Court stated:

Where, as here, the Government uses a device that is not in general public use, to explore details of the home that would previously have been unknowable without physical intrusion, the surveillance is a "search" and is presumptively unreasonable without a warrant.

  The Kattaria court concludes that a "warrant" as used in Kyllo could be based on the Terry stop reasonable suspicion standard. But I don't think there is much of a reason to think this: The Kyllo Court referred explicitly to a warrant, and the word "warrant" is normally used to mean a probable cause warrant. The Kattaria court notes that there are some kinds of warrants that have been upheld that are not based on probable cause to believe a crime has been committed, such as administrative warrants. That's true, but they are all in the context of "special needs" cases arising outside the context of criminal investigations. This case is not a special needs case, it's a straightforward drug case. I have never heard of a Terry-stop "warrant," and I think the Court is being rather creative in inventing one. I can understand the policy argument for the new rule, but I think it's hard to square with existing precedents.

  As best I can tell, this decision creates a split with the Ninth Circuit but is not itself certworthy; the Kattaria court added an alternative holding that probable cause existed even if it wasn't needed, effectively insulating this particular case from Supreme Court review. But I suspect we'll see this issue in another case, and eventual Supreme Court review seems reasonably likely. Stay tuned. Thanks to Howard for the link.

  UPDATE: This was amended a bit after I realized that one of my arguments misread a line in Kyllo.

  ANOTHER UPDATE: Commenter Gustav points out this very relevant passage from Griffin v. Wisconsin:

JUSTICE BLACKMUN's dissent would retain a judicial warrant requirement, though agreeing with our subsequent conclusion that reasonableness of the search does not require probable cause. This, however, is a combination that neither the text of the Constitution nor any of our prior decisions permits. While it is possible to say that Fourth Amendment reasonableness demands probable cause without a judicial warrant, the reverse runs up against the constitutional provision that "no Warrants shall issue, but upon probable cause." Amdt. 4. The Constitution prescribes, in other words, that where the matter is of such a nature as to require a judicial warrant, it is also of such a nature as to require probable cause. Although we have arguably come to permit an exception to that prescription for administrative search warrants,[fn 4] which may but do not necessarily have to be issued by courts, we have never done so for constitutionally mandated judicial warrants. There it remains true that "[i]f a search warrant be constitutionally required, the requirement cannot be flexibly interpreted to dispense with the rigorous constitutional restrictions for its issue." Frank v. Maryland, 359 U.S. 360, 373 (1959). JUSTICE BLACKMUN neither gives a justification for departure from that principle nor considers its implications for the body of Fourth Amendment law.

Oddly, the Kattaria court does not cite Griffin. Instead, the Eighth Circuit cites its own precedent in United States v. Lucas, No. 05-2165, slip op. at 8-9 (8th Cir. Aug. 23, 2007) (en banc), for the view that "the Court has upheld administrative warrants as reasonable without a showing of probable cause in various contexts." But the Kattaria court ignores the key passage from Griffin in which the Court expressly limits the use of non-pc warrants outside the context of administrative warrants.