Pres­i­dent Don­ald Trump on Mon­day re­it­er­at­ed that he’d like to send U.S. cit­i­zens who com­mit vi­o­lent crimes to prison in El Sal­vador, telling that coun­try’s pres­i­dent, Nay­ib Bukele, that he’d “have to build five more places” to hold the po­ten­tial new ar­rivals.

Trump’s ad­min­is­tra­tion has al­ready de­port­ed im­mi­grants to El Sal­vador’s no­to­ri­ous mega-prison CE­COT, known for its harsh con­di­tions. The pres­i­dent has al­so said his ad­min­is­tra­tion is try­ing to find “le­gal” ways to ship U.S. cit­i­zens there, too.

Trump on Mon­day in­sist­ed these would just be “vi­o­lent peo­ple,” im­ply­ing they would be those al­ready con­vict­ed of crimes in the Unit­ed States, though he’s al­so float­ed it as a pun­ish­ment for those who at­tack Tes­la deal­er­ships to protest his ad­min­is­tra­tion and its pa­tron, bil­lion­aire Elon Musk. But it would like­ly be a vi­o­la­tion of the U.S. Con­sti­tu­tion for his ad­min­is­tra­tion to send any na­tive-born cit­i­zen forcibly in­to an over­seas prison. In­deed, it would like­ly even vi­o­late a pro­vi­sion of a law Trump him­self signed dur­ing his first term.

Here’s a look at the no­tion of send­ing U.S. cit­i­zens to prison in a for­eign coun­try, why it’s like­ly not le­gal and some pos­si­ble le­gal loop­holes.

If it’s le­gal to do to im­mi­grants, why not cit­i­zens?

Im­mi­grants can be de­port­ed from the Unit­ed States, while cit­i­zens can­not. De­por­ta­tion is cov­ered by im­mi­gra­tion law, which does not ap­ply to U.S. cit­i­zens. Part of be­ing a cit­i­zen means you can­not be forcibly sent to an­oth­er coun­try.

Im­mi­grants can be re­moved, and that’s what’s been hap­pen­ing in El Sal­vador. The coun­try is tak­ing both its own cit­i­zens that the Unit­ed States is send­ing as well as those from Venezuela and po­ten­tial­ly oth­er coun­tries that will not take their own cit­i­zens back from the U.S. The Venezue­lans sent there last month had no op­por­tu­ni­ty to re­spond to ev­i­dence against them or ap­pear be­fore a judge.

That’s the deal the Trump ad­min­is­tra­tion signed with Bukele. The U.S. has sent peo­ple to El Sal­vador, Cos­ta Ri­ca, Pana­ma and else­where even when they are not cit­i­zens of those coun­tries. But, un­der in­ter­na­tion­al agree­ments, peo­ple can­not be sent to coun­tries where they are like­ly to be per­se­cut­ed or tor­tured.

Why does the Trump ad­min­is­tra­tion want to send peo­ple to El Sal­vador?

Bukele calls him­self “the world’s coolest dic­ta­tor” and has cracked down on hu­man rights dur­ing his ad­min­is­tra­tion. He’s al­so turned El Sal­vador from one of the world’s most vi­o­lent coun­tries in­to a fair­ly safe one. Trump has em­braced that ex­am­ple, in­clud­ing dur­ing the Oval Of­fice vis­it Mon­day.

Send­ing im­mi­grants from coun­tries like Venezuela to El Sal­vador sends a mes­sage to would-be mi­grants else­where about the risks of try­ing to make it to — or stay in — the Unit­ed States.

There’s a sec­ond ben­e­fit to the ad­min­is­tra­tion: Peo­ple sent to El Sal­vador are out­side the ju­ris­dic­tion of Unit­ed States courts. Judges, the ad­min­is­tra­tion ar­gues, can’t or­der some­one sent to El Sal­vador to be re­leased or shipped back to the U.S. be­cause the U.S. gov­ern­ment no longer has con­trol of them.

It’s a po­ten­tial le­gal loop­hole that led Supreme Court Jus­tice So­nia So­tomay­or to is­sue a grim warn­ing in her opin­ion in a 9-0 U.S. Supreme Court find­ing that the ad­min­is­tra­tion could not fly al­leged Venezue­lan gang mem­bers to El Sal­vador with no court hear­ing, even af­ter Trump in­voked an 18th cen­tu­ry law last used dur­ing World War II to claim wartime pow­ers.

“The im­pli­ca­tion of the Gov­ern­ment’s po­si­tion is that not on­ly nonci­t­i­zens but al­so Unit­ed States cit­i­zens could be tak­en off the streets, forced on­to planes, and con­fined to for­eign pris­ons with no op­por­tu­ni­ty for re­dress,” So­tomay­or warned. She was writ­ing to dis­sent from the ma­jor­i­ty tak­ing the case from the fed­er­al judge who had ini­tial­ly barred the ad­min­is­tra­tion from any de­por­ta­tions and had or­dered planes en route to El Sal­vador turned around — an or­der the ad­min­is­tra­tion ap­par­ent­ly ig­nored.

A sec­ond case high­lights the risks of send­ing peo­ple to El Sal­vador. The ad­min­is­tra­tion ad­mits it sent a Mary­land man, Kil­mar Abrego Gar­cia, er­ro­neous­ly to El Sal­vador. A Sal­vado­ran im­mi­grant, Abrego Gar­cia, who has not been charged with a crime, had an or­der against de­por­ta­tion but was shipped to CE­COT any­way. On Mon­day Bukele and Trump scoffed at the idea of send­ing him back, even though the U.S. Supreme Court or­dered the ad­min­is­tra­tion to “fa­cil­i­tate” his re­turn.

Wait, so can they send cit­i­zens to El Sal­vador?

Noth­ing like this has ever been con­tem­plat­ed in U.S. his­to­ry, but it seems un­like­ly. There are oth­er le­gal bar­ri­ers be­sides the fact that you can­not de­port U.S. cit­i­zens. The Unit­ed States does have ex­tra­di­tion treaties with sev­er­al coun­tries where it will send a cit­i­zen ac­cused of a crime in that coun­try to face tri­al there. That ap­pears to be the on­ly ex­ist­ing way a U.S. cit­i­zen can be forcibly re­moved from the coun­try un­der cur­rent law.

The Con­sti­tu­tion al­so pro­hibits “cru­el and un­usu­al pun­ish­ment,” and one of CE­COT’s sell­ing points is that con­di­tions there are far harsh­er than in pris­ons in the U.S. As not­ed above, fed­er­al courts have no ju­ris­dic­tion there, and that may de­prive peo­ple sent there of the con­sti­tu­tion­al guar­an­tee of due process of law.

She not­ed that even if the ad­min­is­tra­tion tries to trans­fer fed­er­al pris­on­ers there, ar­gu­ing they’re al­ready in­car­cer­at­ed, it could run afoul of the First Step Act that Trump him­self cham­pi­oned and signed in 2018. The pro­vi­sion re­quires that the gov­ern­ment try to house fed­er­al in­mates as close to their homes as pos­si­ble so their fam­i­lies can vis­it them — and in­deed trans­fer any­one housed far­ther than 500 miles from their home to a clos­er fa­cil­i­ty.

One last loop­hole?

There is one po­ten­tial loop­hole that the ad­min­is­tra­tion could use to send a small group of cit­i­zens to El Sal­vador. They can try to strip the cit­i­zen­ship of peo­ple who earned it af­ter im­mi­grat­ing to the Unit­ed States.

Peo­ple who were made U.S. cit­i­zens af­ter birth can lose that sta­tus for a hand­ful of of­fens­es, like fund­ing ter­ror­ist or­ga­ni­za­tions or ly­ing on nat­u­ral­iza­tion forms. They would then re­vert to green card hold­ers, and would be po­ten­tial­ly el­i­gi­ble for de­por­ta­tion if con­vict­ed of oth­er, se­ri­ous crimes.

That’s a small, but re­al, pool of peo­ple. Per­haps the most sig­nif­i­cant thing about it is that it would re­quire loss of cit­i­zen­ship first. In oth­er words, there’s still like­ly no le­gal way to force a cit­i­zen out of the coun­try. But a few could end up in le­gal jeop­ardy any­way.