Trump Says U.S. Will Hold Migrants at Guantánamo Bay

President Trump on Wednesday ordered his administration to prepare to house tens of thousands of “criminal aliens” at the Navy base at Guantánamo Bay, the latest prong in his widening crackdown on immigration.

Mr. Trump did not offer details on how the plan would take shape, but he instructed the Departments of Defense and Homeland Security to get the site ready.

“We have 30,000 beds in Guantánamo to detain the worst criminal illegal aliens threatening the American people,” he said. “Some of them are so bad we don’t even trust the countries to hold them, because we don’t want them coming back, so we’re going to send them out to Guantánamo.”

He said the move would “double our capacity immediately,” adding that Guantánamo was a “tough place to get out of.”

In recent weeks, about 40,000 immigrants have been held in private detention centers and local jails around the country as funding constraints have limited the number of detention sites.

Adding 30,000 beds would dramatically expand the government’s detention capacity. A site on the 45-square-mile base could hold those 30,000 deportees. That site is on the opposite side of the body of water called Guantánamo Bay from the Pentagon’s prison for terrorism suspects.

Successive administrations have prepared fields on a remote section, near the airfield but far from the population center, to accommodate tens of thousands of migrants in a sprawling tent city.

The infrastructure was set up starting in the mid-2000s to shelter Cubans and others from the region who had been intercepted while fleeing their country. The Clinton administration had tasked Guantánamo with the role in the 1990s. It was designed as a humanitarian relief operation.

It was not immediately clear how such an operation under Mr. Trump would be staffed, secured and what rights, if any, the deportees would have at Guantánamo Bay. Civil liberties groups expressed concerns.

Vincent Warren, executive director of the Center for Constitutional Rights, said Mr. Trump’s order sent a dark message that “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.”

Mr. Trump’s memo called for expanding the Migrant Operations Center, which currently occupies a small former barracks that has had capacity for up to 120 migrants but in recent years held at most dozens at a time. It is near empty fields that could be transformed into a tent city.

Tom Homan, Mr. Trump’s border czar, told reporters outside the White House on Wednesday that certain migrants could be flown to the island, and that the operation would be run by Immigration and Customs Enforcement, known as ICE.

“The worst of the worst, the significant public safety threats we can fly them,” he said.

U.S. military and Homeland Security forces have periodically rehearsed how to handle a migrant crisis at the site.

In the 1990s, the base was overwhelmed by more than 45,000 people fleeing crises in both Haiti and Cuba. They were housed in crude tent cities on the populated side of the base, including on the current site of the Pentagon’s detention facility for detainees in the war against terrorism. Today, that facility houses 15 prisoners and is staffed by 800 troops and civilians.

Starting with the George W. Bush administration, the government created a new footprint for a future humanitarian relief operation on the mostly empty side of the base.

During semiannual drills for a humanitarian relief operation, the Southern Command typically flew in a few hundred soldiers from Fort Sam Houston in San Antonio to play different roles.

The proposed site of the tent camps could be surrounded with barbed wire, like the military did for the tent camps of the 1990s, which housed both families and single men.

Deborah Fleischaker, an ICE official during the Biden administration, said that detaining immigrants at the base would be particularly difficult.

“Gitmo is very small and very remote,” she said, using the military’s nickname for the site. “Moving materials and people in and out would be a logistical nightmare. And the makeup of who would be held there is very important. Only men? Women and children? If women and children are there, the housing challenges become even more difficult.”

In the last week, the Trump administration has undertaken a sweeping blitz on immigration, including arrests in communities across the country. Mr. Trump has promised to conduct a historic mass deportation effort, but such a plan would need expanded detention capabilities and more resources.

Since the late 1990s, around 500 migrants have been resettled in third countries from Guantánamo.

“Guantánamo is a black hole designed to escape scrutiny and with a dark history of inhumane conditions. It is a transparent attempt to avoid legal oversight that will fail,” said Lucas Guttentag, a Justice Department official in the Biden administration who once led the lawsuit over Haitian refugees being held at the site.

Erica L. Green contributed reporting from Washington.