Law enforcement agents at the border or screening passengers from an international plane flight have the authority to question and search people who are entering the country (citizens and non-citizens alike) to a virtually unlimited extent.

Law enforcement agents can set up fixed checkpoints on roads inside the United States, but near to the border, and stop cars for questioning. The officers don’t need any proof of wrongdoing to question the car’s occupants, but to search the car they must have probable cause or consent.

Law enforcement agents can also drive around in the vicinity of the border and stop cars that the officers reasonably suspect contain people who’ve entered the country illegally. The officers can question the occupants of the car, but they can’t search the vehicle—unless a rationale for a warrantless search applies.Law enforcement officers can search without a warrant under a wide variety of circumstances. Among these, there’s only one situation in which you have any chance of preventing the intrusion—and that’s by saying “I don’t consent” when the police ask whether they can search. This is a powerful tool for using your civil rights, as important as remaining silent and asking to see a lawyer.

The right that upholds your privacy is the Fourth Amendment to the U.S. Constitution, which protects your body, home, papers and possessions from unreasonable searches and seizures. Law enforcement agents are forbidden to violate your privacy unless they have a specific legal justification, such as executing a valid warrant or following a fleeing suspect in hot pursuit. If officers are asking your permission to search, that means that they don’t honestly believe they have one of these lawful excuses—so they’re hoping that you’ll foolishly give up your rights and consent to the search.

An officer’s request to search often sounds like an informal order, as in:

Why don’t you show me what you’ve got in that bag?

There’s been a report of an incident near here. I want to come in and take a look around.

Let’s see some i.d.

The officer won’t point out that you have the option to refuse. So it’s up to you not to open or unpack anything, until you’ve verified that the police are giving you an actual order. You can say, “Are you just asking me, or are you ordering me?” Either the cop will give up, or he’ll specifically order you to comply with the search.