Pretty horrible story of a US journalist who had his computer and phone searched at the border when returning to the US from Mexico.
After I gave him the password to my iPhone, Moncivias spent three hours reviewing hundreds of photos and videos and emails and calls and texts, including encrypted messages on WhatsApp, Signal, and Telegram. It was the digital equivalent of tossing someone’s house: opening cabinets, pulling out drawers, and overturning furniture in hopes of finding something—anything—illegal. He read my communications with friends, family, and loved ones. He went through my correspondence with colleagues, editors, and sources. He asked about the identities of people who have worked with me in war zones. He also went through my personal photos, which I resented. Consider everything on your phone right now. Nothing on mine was spared.
Pomeroy, meanwhile, searched my laptop. He browsed my emails and my internet history. He looked through financial spreadsheets and property records and business correspondence. He was able to see all the same photos and videos as Moncivias and then some, including photos I thought I had deleted.
If you are a U.S. citizen, border agents cannot stop you from entering the country, even if you refuse to unlock your device, provide your device password, or disclose your social media information. However, agents may escalate the encounter if you refuse. For example, agents may seize your devices, ask you intrusive questions, search your bags more intensively, or increase by many hours the length of detention. If you are a lawful permanent resident, agents may raise complicated questions about your continued status as a resident. If you are a foreign visitor, agents may deny you entry.
The most important piece of advice is to think about this all beforehand, and plan accordingly.