Estelle, who’s long held permanent resident status in the U.S., is a veteran at navigating the reentry process when she returns from visiting relatives in her native France.

But on her most recent trip through customs in mid-March, officers detained the 57-year-old Lawrence, Kansas, resident for 30 hours, forced her to spend the night in a holding cell on a concrete slab and threatened her with deportation.

Why? Because she acknowledged under questioning by customs officers that she’d once voted in a local election, despite not being a U.S. citizen. A small number of cities in the U.S. allow noncitizens to vote in local elections, but Lawrence is not one of them. Kansas and federal law both require U.S. citizenship to register to vote.

Immigration and election experts say her case, which hasn’t previously been reported, marks a new escalation in the Trump administration’s efforts to find and prosecute instances of noncitizen voting, despite voluminous evidence showing it is rare. (Estelle asked that her last name not be used because of safety concerns.)

Historically, U.S. Customs and Border Protection has played no part in election-fraud investigations. But the transcript of Estelle’s interview, which was provided to ProPublica by her attorney, makes clear that the agency had flagged her for special scrutiny and that officers knew her voting history. Estelle told the officer during questioning that she thought she could vote in local elections because a state motor vehicles department employee had told her when she renewed her driver’s license that she was eligible.

Kerry Doyle, a deputy general counsel for the Department of Homeland Security in the Biden administration, said she’d never heard of someone being detained at a port of entry on suspicion of voting illegally.

“It took them a whole lot of energy and effort to sift through all these things to........

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