President Donald Trump’s effort to transfer migrants from the United States to the Naval base at Guantanamo Bay, Cuba is being buffeted by concerns inside and outside his administration.

The Trump administration has flown all remaining migrants held at the Guantánamo Bay Naval Station back to the United States, a Defense Department official confirmed

The U.S. military has started making plans to draw down personnel deployed to the naval base in the coming weeks.

Trump’s failed operation to send migrants to Guantanamo Bay has already burned through at least $16 million, The Wall Street Journal reports. That figure does not include the wildly expensive flights—it costs more than $20,