US Secretary of State Marco Rubio could not have been more complimentary about the deal he struck with the president of El Salvador on Monday.

The Trump administration’s top diplomat appeared delighted yet stunned by the fact that President Nayib Bukele should have “agreed to the most unprecedented, extraordinary, extraordinary migratory agreement anywhere in the world”.

Bukele had offered to take in people deported from the US, regardless of their nationality, and house them in El Salvador’s mega-jail.

“We can send them and he will put them in his jails,” Rubio said.

While that was already a win for President Donald Trump, whose priority has been to speed up the removal of undocumented migrants from the US, the real surprise came in the part of the deal Rubio mentioned next.

“He [Bukele] has also offered to do the same for dangerous criminals currently in custody and serving their sentences in the United States even though they’re US citizens or legal residents,” Rubio said.

The Salvadorean leader confirmed that he had “offered the United States of America the opportunity to outsource part of its prison system”.

He clarified that El Salvador would be “willing to take in only convicted criminals” and that his government would do so “in exchange for a fee”.

Bukele also revealed where he would house those deported from the US: “our mega-prison”.

The mega-jail, also known as Cecot (short for Terrorism Confinement Centre), has become emblematic of Bukele’s iron-fist approach to crime and punishment.

The maximum-security prison, one of the largest in Latin America, opened in January 2023 and can house 40,000 inmates, according to government figures.

Inmates are confined to windowless cells, sleep on bare metal bunks and are constantly monitored by armed guards – some of whom watch over them from atop the lattice ceiling.

BBC News Mundo’s Leire Ventas, who was allowed to take an official tour of the facility last year after the BBC had repeatedly asked for access, described how temperatures in the cells would reach 35C.

Take a look at graphics and maps of the mega-jail

With access to the prison severely restricted and journalists only allowed on occasional and carefully choreographed official tours, the number of inmates per cell is not clear.

Some rights groups put it at 80 prisoners while others say it can go up to more than 150.

Asked by our journalist what the maximum capacity was, the prison’s director responded “where you can fit 10 people, you can fit 20”.

Prisoners are locked up inside their cells 24 hours a day – except for 30 minutes of group exercise in a windowless corridor.

The layout of the jail is no coincidence.

Following a particularly bloody weekend in 2022, when more than 70 people were killed in the small Central American nation, President Bukele wrote on social media: “Message for the gangs: because of your actions, your ‘homeboys’ will not be able to see a ray of sunshine.”

Building of the Cecot mega-jail started shortly afterwards.

Conditions at the facility and the treatment of inmates has come under severe criticism from human rights groups.

Miguel Sarre, a former member of the United Nations Subcommittee for the Prevention of Torture, has described it as a “concrete and steel pit”.

So could the Trump administration send US citizens there?

On Tuesday, Trump told reporters he would embrace the idea but questioned its legality.

“If we had the legal right to do it, I would do it in a heartbeat,” he said during an executive order signing ceremony in the Oval Office. “I don’t know if we do or not.

“We’re looking at that right now, but we could make deals where we’d get these animals out of our country.”

But any attempt to deport US citizens or people lawfully resident in the US to a foreign jail is bound to face legal challenges.

US citizens who were born in the United States enjoy legal protection from deportation.

There are some cases, however, in which naturalised citizens – those who were not born in the US and who obtained US citizenship after birth through a legal process – can have their citizenship revoked.

This tends to occur when the person in question used fraud to obtain the citizenship in the first place.

Alex Cuic, an immigration lawyer and professor at Case Western Reserve University in Ohio, told the BBC that naturalised US citizens suspected of ties to criminal gangs or terrorist organisations – such as the Tren de Aragua criminal gang or the Mara Salvatrucha, known as MS-13 – could also, in theory, be stripped of US citizenship.

“If they find out you were a member of any group that persecuted or threatened to persecute others, they can try to denaturalise you,” Mr Cuic added.

“So, if you had gang ties and never disclosed them, they could use that as a reason to denaturalise you.”

Once a person has been “denaturalised”, they are at risk of deportation.

Mr Cuic pointed out that any such move would have to be preceded by a “formal court process” conducted in a federal court.

But the lawyer warned that “citizenship is not something that is definitively forever if you are naturalised”.

He stressed, though, that he had “never heard” of cases of natural-born US citizens being sent abroad for imprisonment for crimes committed and prosecuted in the US.

Shev Dalal-Dheini, the director of government relations for the American Immigration Lawyers Association, similarly said that she had “never heard of such a suggestion” as sending American citizens to serve US prison sentences overseas.

While she acknowledged that there were various scenarios in which naturalised US citizens could lose their citizenship, she said that “you can’t denaturalise a natural-born citizen”.

The status of lawful permanent residents in the US, however, is more precarious than that of US citizens.

They can be deported if they violate certain provisions of the Immigration and Nationality Act, which include committing drug offences, violent crimes or crimes such as theft, fraud or assault.

Like naturalised citizens, they can also be deported if they obtained their residency through fraud.

Permanent lawful residents who are involved in terrorism, espionage or any activity threatening US national interest could also be deported.

This last point is important in light of the executive order President Trump issued on his inauguration day in which he designated drug cartels as “foreign terrorist organisations”.

Two criminal organisations named in the executive order, Tren de Aragua and MS-13, were also mentioned last week by Trump’s special envoy for Latin America, Mauricio Claver-Carone.

Speaking at a briefing about Marco Rubio’s trip to El Salvador, Claver-Carone not only praised Bukele’s handling of the MS-13 – a gang which is deeply rooted in El Salvador and has long terrorised its citizens – but also said that Bukele could offer the answer on how to deal with the Venezuelan gang Tren de Aragua.

Claver-Carone also appeared to argue that the mere prospect of being sent to a Salvadoran jail could drive Venezuelan gang members back to their homeland.

“I bet they’re going to want to go back to Venezuela instead of dealing with the Mara prisons in El Salvador,” he said of members of the Tren de Aragua gang.

Rubio, too, seemed to stress that the Trump administration would first and foremost want to send members of these two notorious gangs to El Salvador’s prisons.

“Any unlawful immigrant and illegal immigrant in the United States who is a dangerous criminal – MS-13, Tren de Aragua, whatever it may be – he has offered his jails,” Rubio said after his talks with Bukele.

While it is by no means clear who – if anyone – will be sent from the US to El Salvador’s mega-prison, what is certain is that with his “unprecedented offer of friendship”, Bukele has landed firmly in Trump’s favour at a time when relations between the US and its neighbours have been rocked by the US president’s threats to impose tariffs on their goods.

With additional reporting by the BBC’s Bernd Debusmann Jr in Washington, DC.

Follow the twists and turns of Trump’s second presidential term with North America correspondent Anthony Zurcher’s weekly US Politics Unspun newsletter. Readers in the UK can sign up here. Those outside the UK can sign up here.