On June 20, U.S. Customs and Border Protection (CBP) agents in Maine set up a checkpoint on Interstate 95, approximately 80 miles from the Canadian border, according to The New York Times. The Times reported that for 11 hours that day, the agents asked every motorist for proof that they are U.S. citizens. “If you want to continue down the road,” one agent told a pair of reporters from the Bangor Daily News, “we need to know what country you’re a citizen of.”

Elsewhere in the country, including Florida and New York, CBP agents have recently conducted impromptu sweeps on commercial buses, asking passengers for proof of citizenship.

These checkpoints and bus sweeps aren’t new — CBP has been conducting them for years — but they seem to have become more common since President Donald Trump was inaugurated. In May, the American Civil Liberties Union (ACLU) of Maine filed a lawsuit against CBP and the Department of Homeland Security, claiming that there have been “increasing reports” of CBP “stopping bus passengers to check their citizenship status without a warrant, reasonable suspicion, or probable cause,” according to the lawsuit.

Federal laws allow CBP to ask people for proof of citizenship if they’re “within a reasonable distance from any external boundary of the United States,” which is defined as 100 miles from any part of the U.S. border or coastline, according to the ACLU. This area is commonly referred to as the “100-mile border zone.” It includes big cities like Chicago, New York City, and Los Angeles, as well as entire states like Florida and Michigan, as shown in a recent data analysis by an MIT researcher.

U.S. law does not require citizens to carry papers proving their citizenship, according to the ACLU. (There have, however, been several cases of U.S. citizens being arrested and even deported by ICE.) People who have certain forms of legal status, including a green card, are required to carry this documentation at all times. As Vox’s Alexia Fernández Campbell explained, CBP isn’t looking for proof that you’re a citizen — what they’re really looking for is proof that you aren’t.

CBP may be able to ask you for your papers, but that doesn’t mean you’re always required to comply. “If an immigration official stops you, you can choose to remain silent,” Adriana Piñon, policy counsel and senior staff attorney at the ACLU of Texas, tells Teen Vogue. She says that choosing to remain silent or asking to speak to a lawyer could prolong your interaction with the officers, but it could also shield you from deportation. “When one is speaking with a law enforcement officer — and an immigration officer in particular — it’s very important to remember that anything you say can be used against you. If what you say gives them grounds to arrest you, then they will or they can.”

“Exercising your right to remain silent should not constitute reasonable suspicion for any crime,” Piñon says. “If you exercise your right to remain silent and they arrest you, that would violate your basic constitutional rights.” Those rights apply even if you’re not a U.S. citizen — even if you’re undocumented, she says. But, Piñon noted, there are some people who are required to show CBP their documents, such as green card holders or people who are on temporary student visas. She suggested that anyone who is unsure about whether they are considered “non-immigrants” should contact an immigration attorney.