As of 2018, Border Patrol (CBP) authorities have the authority to inspect your cell phone should they deem it necessary. This includes the electronic devices of U.S. citizens as well as foreigners.
However, many considered these measures invasive and there were many reports, mostly from U.S. citizens. In fact, between 2018 and 2019, 41 thousand travelers' cell phones were inspected.
It is because of the above that yesterday, the U.S. Court of Appeals for the Ninth Circuit made the decision to overturn the ability of federal agents to seize personal electronic devices.
It is important to note that the court's decision only includes ports of entry, airports and ports of lay over in the western United States within the jurisdiction of the U.S. Court of Appeals for the Ninth Circuit.
Here is this map, where what you see in light yellow are the areas that include this annulment:
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