“We’re going to send them out to Guantánamo,” Trump said at the signing of the Laken Riley Act.

He later signed a presidential memorandum and said he’d direct federal officials to get facilities ready to receive criminal immigrants in the US illegally. Border czar Tom Homan said U.S. Immigration and Customs Enforcement would run the facility. Still, details of the plan weren’t immediately clear.

Here’s a look at the U.S. naval base, widely known as “Gitmo,” and its history:

How does the US government use the base at Guantánamo Bay?

While the U.S. naval base in Cuba is best known for the suspects brought in after the Sept. 11, 2001, attacks, it has a small, separate facility used for decades to hold migrants.

The Migrant Operations Center is used for people intercepted trying to illegally reach the U.S. by boat. Most are from Haiti and Cuba.

The center takes up a tiny part of the base, includes just a handful of buildings and has nowhere near the capacity to house the 30,000 people Trump said could be sent there.

“We’re just going to expand upon that existing migrant center, Homan told reporters.

The migrant detention center operates separately from the military’s detention center and courtrooms for foreigners detained under President George W. Bush during what that administration called its “war on terror.” That facility houses 15 detainees, including accused 9/11 mastermind Khalid Sheikh Mohammed. That’s down from its peak of nearly 800.

Who will be held at Guantánamo?

The migrant detention facilities at Guantánamo will be used for “the worst of the worst,” administration officials said.

Homeland Security Secretary Kristi Noem and Homan both used the phrase when speaking to reporters outside the White House.

A White House statement was less specific, saying the expanded facility would “provide additional detention space for high-priority criminal aliens unlawfully present in the United States, and to address attendant immigration enforcement needs.”

An administration official, speaking on condition of anonymity because they weren’t authorized to speak publicly on the matter, said it would be used to house “dangerous criminals” and people who are “hard to deport.”

A number of countries refuse to accept some immigrants the U.S. tries to deport.

Trump has repeatedly spoken about the dangers Americans face from the estimated 11 million immigrants living in the U.S. illegally. While immigrants are regularly charged with committing major crimes, they are a tiny percentage of the overall population. Peer-reviewed academic studies have generally found no link between immigration and violent crime, though conclusions vary.

What else is known about the Migrant Operations Center?

Not much. The nonprofit International Refugee Assistance Project said in a report last year that people are held in “prison-like” conditions. It said they were “trapped in a punitive system” indefinitely, with no accountability for the officials running it.

Deepa Alagesan, a senior supervising attorney with the group, said Wednesday that they believed it is used to hold a small number of people — “in the double digits,” she estimated.

The prospect of using it for far more immigrants worried her.

“It’s definitely a scary prospect,” she said.

Does the U.S. have sufficient detention space for Trump’s plans?

Trump has vowed to deport millions of people living illegally in the U.S., but the current Immigration and Customs Enforcement budget only has enough funds to detain about 41,000 people.

ICE detains immigrants at its processing centers and privately operated detention facilities, along with local prisons and jails. It has no facilities geared toward the detention of families, who account for roughly one-third of arrivals on the southern U.S. border.

During Trump’s first term, he authorized the use of military bases to detain migrant children. In 2014, then-President Barack Obama temporarily relied on military bases to detain immigrant children while ramping up privately operated family detention centers to hold many of the tens of thousands of Central American families caught illegally crossing the border.

U.S. military bases have been used repeatedly since the 1970s to accommodate the resettlement of waves of immigrants fleeing Vietnam, Cuba, Haiti, Kosovo and Afghanistan.

What do lawyers in Sept. 11 cases say?

The decision to send immigrants to Guantánamo “should horrify us all,” said a legal advocacy group that since the Sept. 11 attacks has represented dozens of men detained at the base.

Trump’s order “sends a clear message: migrants and asylum seekers are being cast as the new terrorist threat, deserving to be discarded in an island prison, removed from legal and social services and supports,” Vince Warren, the executive director of the New York-based Center for Constitutional Rights, said in a statement.

What is the reaction in Cuba?

The U.S. has leased Guantánamo from Cuba for more than a century. Cuba opposes the lease and typically rejects the nominal U.S. rent payments.

Government officials criticized the news Wednesday, with President Miguel Díaz-Canel deeming the decision “an act of brutality” on X and describing the base as “located in illegally occupied #Cuba territory.”

Foreign Minister Bruno Rodríguez said on X, “The US government’s decision to imprison migrants at the Guantánamo Naval Base, in an enclave where it created torture and indefinite detention centers, shows contempt for the human condition and international law.”