Searches and Seizures at the Border and the Fourth Amendment [Updated March 30, 2021] [open pdf - 2MB]
From the Summary: "Congress has broad authority to regulate persons or items entering the United States, an authority that is rooted in its power to regulate foreign commerce and to protect the integrity of the nation's borders. Exercising this authority, Congress has established a comprehensive framework that authorizes federal law enforcement officers to inspect and search persons and property at the border to ensure that their entry conforms with governing laws, including those relating to customs and immigration."
CRS Report for Congress, R46601
Congressional Research Service: https://crsreports.congress.gov/