Traditionally, the Department of Homeland Security and the U.S. Border Patrol searches at the border didn’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. When the courts first applied the Fourth Amendment to border searches of computers, they held that searches of computers were ordinary searches that required no suspicion. As a result, border agents traditionally have been free to seize anyone’s cell phone and examine anything and everything stored within the phone with no reason required.
Now a new case before the Fourth Circuit has resulted in a ruling that at least some suspicion is required for a forensic search of a cell phone seized at the border.
Details may be found in an article by Orin Kerr in the Reason.com web site at http://reason.com/volokh/2018/05/09/important-fourth-circuit-ruling-on-cell.