International travel for most Americans requires the proper documents and a lot of patience to get to the front of the immigration line and back into the country. But if Customs and Border Protection pulls you off to the side for a secondary screening, here’s what you need to know.

The big picture: The 4th Amendment prohibits unreasonable searches and seizures, but it doesn’t apply at the U.S. border, says Georgetown law professor M. Tia Johnson, a former assistant secretary at the Department of Homeland Security. 

• Expectation of privacy is generally lower when traveling, and officials need to enforce immigration and customs laws.
• A U.S. citizen cannot refuse a secondary inspection, according to Sophia Cope, a senior attorney at the Electronic Frontier Foundation.
• It’s best to comply with officials’ requests, but Cope advises discretion if it feels like some questions are going too far.
• CBP can search an American traveler’s luggage and vehicle without a warrant at the border. 

Why it matters: A U.S. citizen cannot be denied entry. 

• U.S. citizens must be admitted, says Cope.
• Green card holders should also be allowed entry back into the U.S. as long as they haven’t been outside of the U.S. for more than a year.
• However, American travelers can find themselves undergoing secondary inspection if they don’t have the proper travel documents, their passport has expired or they’re on a no-fly list, according to Johnson.

There is no limit on how long CBP officials can hold a U.S. citizen, but they usually don’t hold people for more than several hours or overnight because courts may look upon the detention as unreasonable, per Cope.

CBP may ask to see personal electronic devices, according to a CBP spokesperson.

• The courts are still debating whether CBP officials can look at the contents, says Cope.
• CBP can inspect electronic devices for hidden contraband.
• A citizen can refuse to give up the passcodes to their devices, but CBP can then confiscate the items. 
• One circuit court has said CBP needs a warrant to analyze deeply or copy the contents of an electronic device, according to Johnson.

Driving the news: Francisco Erwin Galicia, a U.S. citizen by birth, was traveling through southern Texas for college soccer tryouts when CBP stopped him, reports the Dallas Morning News. He was detained for nearly 3 weeks because CBP questioned his citizenship status and the validity of his documents. 

• While Cope says CBP generally doesn’t hold people for more than a few hours, she says “immigration enforcement in the context of suspecting someone is already in the country illegally is its own legal animal