If you're a US citizen, I believe they can seize your device indefinitely, and detain you for up to two days. They are required to let you eventually back into the country though.

(If you're not a citizen, all bets are off)

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NoImmatureAdHom a day ago | parent [-]

Here's what the ACLU says:

https://www.aclu.org/news/privacy-technology/can-border-agen...

I think the 48h detention is across the board (without a judge involved, border or not the border). ACLU says device seizure is up to 5 days barring "extenuating circumstances", whatever that means.

	▲	nerdsniper a day ago \| parent [-]
		The same article says they can fairly arbitrarily do repeated 7-day extensions. And: > We’ve received reports of phones being held for weeks or even months.