A U.S. military plane flew a group of migrants from El Paso, Texas, to the U.S. Navy base in Guantánamo Bay, Cuba, U.S. officials said Tuesday, a significant step in President Donald Trump’s plan to house tens of thousands of deportees on the base.

“The first flights from the United States to Guantánamo Bay with illegal migrants are underway,” White House press secretary Karoline Leavitt told Fox Business.

The migrants were transferred in an Air Force C-17 transport plane. They arrived at the base Tuesday evening and were expected to be held in an area far from the Pentagon’s prison for terrorism suspects.

The decision is a change in how the United States handles people it deports. The U.S. government has long held migrants it picked up at sea in a facility at Guantánamo Bay, but it has not flown migrants from within the United States to the base.

It also serves as a potent symbol of Trump’s attempts to cut off legal and illegal immigration, a key pillar of his campaign for the presidency and part of his promise for his administration. Last week, Trump ordered the Defense Department and the Department of Homeland Security to prepare to expand Guantánamo Bay for the detention of migrants.

“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.”

Trump said it was also a “tough place to get out of.”

About 300 service members have arrived at Guantánamo Bay to provide security and begin setting up at a new tent city for the migrants. U.S. forces have put up 50 Army green tents inside a chain-link-fence enclosure, adjacent to a barracks-style building called the Migrant Operations Center.

Criticism

Immigrant advocates said sending migrants to Guantánamo was unacceptable.

“The United States has a deplorable history of detaining different groups of people unlawfully at Guantánamo to avoid oversight and the public eye, and this latest chapter is no exception,” Hannah Flamm, the senior director of policy at the International Refugee Assistance Project, said in a statement.

“The Trump administration is engaging in political theater to threaten immigrants with detention at one of the most notorious facilities in the world, with grave implications for the basic rights of individuals and for American rule of law,” she said.

On Sunday, Kristi Noem, the secretary of homeland security, said the facility would be an asset for the agency.

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.