The U.S. government claims the right to eavesdrop at will on a citizen’s email if he or she is writing to someone who lives abroad. Now it wants to be able to use those emails to convict the person sending them of a crime.

That’s what’s happening to Aws Mohammed Younis al-Jayab — and he’s not the only one. The legal basis is the 2008 Amendment Act to the Foreign Intelligence Surveillance Act, which says the government may monitor communications from within the U.S. to foreigners abroad, or vice versa, without first obtaining a warrant to authorize the surveillance.

No court has yet reviewed the law’s constitutionality because until 2013 the government didn’t tell anyone that it had been doing this. The Supreme Court ruled in 2013 that no one had legal standing to challenge the law based merely on the speculation that it might be applied to them.

Jayab is different. The government can charge him with a crime only by using evidence gathered from his intercepted emails. So it’s put him on notice that it intends to rely on material collected without a warrant per the FISA. That gives Jawab standing to challenge the law.

Rightfully, Jayab should win — and the details of his case show why. It’s one thing for the government to intercept communications with foreigners for intelligence-gathering purposes. I would consider that a close debate. But it’s quite another to use those intercepts as evidence at trial. Such use badly erodes protections in the U.S. Constitution’s Fourth Amendment, especially in an era when so much electronic communication spans the globe and where we move across borders frequently.

The alleged facts of Jayab’s case are telling. The Sacramento, California, resident came to the U.S. as a refugee from Iraq in 2012. According to the government, while living in Arizona and Wisconsin, he emailed with jihadis in Syria about going there to fight. The emails indicated he had been there before. And sure enough, in 2014, Jayab traveled to Turkey and from there crossed into Syria to fight alongside several groups, allegedly including Ansar al-Islam, a Kurdish jihadi group that has since merged with Islamic State.

In theory, Jayab could be charged with material support for terror if it could be proven that he really fought with Ansar al-Islam, a designated terrorist group for purposes of that law. But either the government doesn’t want to charge him with that crime for tactical reasons, or it’s afraid it doesn’t have enough direct evidence to prove it.

Instead, the government called Jayab in for an interview with U.S. Citizenship and Immigration Services after his return to the U.S. in 2014. They asked him whether he’d been a member of a rebel group or supported terror. He said no, and claimed to have done nothing more than visit his grandmother in Turkey on the trip.

The government then charged Jayab with the crime of lying to government officials. Its evidence comes from his emails to Syria when he was in the U.S. and responses he received. Obtained under the FISA without a warrant, they are the heart of the government’s case.

To be sure, in the light of the Paris and Brussels attacks, the U.S. needs some way to lock up potential jihadi terrorists who may have been trained abroad to commit attacks here. But relying on warrantless wiretapping isn’t it.

There is a reason that warrantless wiretaps usually are inadmissible in court. They count as searches for purposes of the Fourth Amendment. Requiring a warrant before law enforcement is allowed to listen in is a basic element of our modern right to privacy.

Once we relax that right by allowing evidence obtained in a warrantless search, there’s no easy stopping point. It would seem arbitrary if we could convict one criminal who called an associate outside the U.S., but not another whose associate was inside our borders. Once the former is permitted, we’ll start thinking about how to allow the latter.

What’s more, the Constitution doesn’t say that my privacy stops when I am speaking to someone who happens to be outside the U.S. The founding fathers surely wouldn’t have exempted letters sent from abroad if they were read within the U.S.

The Fourth Amendment provides for the right of “the people” to be free of unreasonable searches and seizures. That need not apply to persons outside the U.S. But it should apply to everyone who is here, and we shouldn’t lose our privacy rights just because we’re talking across borders.

It’s harder to say with certainty that the government should always be barred from such intercepts when they’re made purely for intelligence purposes. Say the government is listening to the phone of someone in Syria and that person gets a call from the U.S. — the government shouldn’t have to stop listening.

But charging a U.S. resident in U.S. court on the basis of warrantless searches of communications while the resident was in the U.S. goes too far. The government should find another way to hold Jayab if he is dangerous, such as charging him with a crime that can be proved without violating his rights — and ours.

Noah Feldman is a Bloomberg View columnist.

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