On Monday, the Supreme Court let stand a March 2013 ruling that established—at least in the Ninth Circuit in the western United States—that extended and sophisticated forensic analysis of a digital device requires a reasonable suspicion of wrongdoing.

The case, United States v. Cotterman, involves an American man who was driving back into the country from Mexico with his wife in 2007 and had his laptop cursorily searched, with a more advanced search then performed at a government facility 170 miles away. The Supreme Court declined to hear Howard Cotterman's appeal of the legality of the extensive search.

As part of a routine check, a border computer system returned a hit for Cotterman—he is a sex offender convicted on several counts, including child molestation in 1992. The agents then searched his car, finding two laptops and three digital cameras, which they also inspected . Those devices had several password-protected files.

The border agents suspected, based in part on the existence of password-protected files, that Cotterman may have been engaged in sex tourism. They interviewed him and his wife and ultimately released them—but kept the laptops and one camera. The laptop was brought to an Immigration and Customs Enforcement (ICE) office in Tucson, Arizona, where an ICE agent performed intensive forensic search on the laptops. On one laptop, the search found 75 images of child pornography in the "unallocated space" of the hard drive, which is where deleted data resides.

Cotterman later argued for suppression of the evidence of child porn that turned up as a result of that forensic analysis. A lower court initially suppressed that evidence, which was then overruled (PDF) by the Ninth Circuit Court of Appeals in March 2013.

As the Ninth Circuit judges wrote:

Although courts have long recognized that border searches constitute a “historically recognized exception to the Fourth Amendment’s general principle that a warrant be obtained,” United States v. Ramsey, 431 U.S. 606, 621 (1977), reasonableness remains the touchstone for a warrantless search. Even at the border, we have rejected an “anything goes” approach. See United States v. Seljan, 547 F.3d 993, 1000 (9th Cir. 2008) (en banc). Mindful of the heavy burden on law enforcement to protect our borders juxtaposed with individual privacy interests in data on portable digital devices, we conclude that, under the circumstances here, reasonable suspicion was required for the forensic examination of Cotterman’s laptop. Because border agents had such a reasonable suspicion, we reverse the district court’s order granting Cotterman’s motion to suppress the evidence of child pornography obtained from his laptop.

Cotterman’s case was referred to in a recent case involving a New York-based student who had his digital devices searched at the US-Canada border. That case, Abidor et al v. Napolitano, was dismissed in December 2013.