Traditionally, searches at the border don't require any suspicion on the theory that the government has a strong sovereign interest in regulating what enters and exits the country. But there is caselaw indicating that some border searches are so invasive that they do require some kind of suspicion.

In the new case, Kolsuz, the Fourth Circuit agrees with the Ninth Circuit that at least some suspicion is required for a forensic search of a cell phone seized at the border. This is important for three reasons. First, the Fourth Circuit requires suspicion for forensic searches of cell phones seized at the border. Second, it clarifies significantly the forensic/manual distinction, which has always been pretty uncertain to me. Third, it leaves open that some suspicion may be required for manual searches, too.

But wait, that's not all. In fact, I don't think it's the most important part of the opinion. The most important part of the opinion comes in a different section, where the Fourth Circuit adds what seems to be a new and important limit on the border search exception: a case-by-case nexus requirement to the government interests that justify the border search exception.

Maybe I'm misreading this passage, but it strikes me as doing something quite new and significant. It scrutinizes the border search that occured to see if the government's cause for searching in this particular case satisified "a 'nexus' requirement" of showing sufficient connection between the search and "the rationale for the border search exception," requiring a link between the "predicate for the search and the rationale for the border exception." In other words, the Fourth Circuit appears to be requiring the government to identify the border-search-related interest justifying that particular search in order to rely on the border search exception.