US Army to control land on Mexico border as part of base, migrants could be detained, officials say

But if the troops are providing security for land that is part of an Army base, they can perform that function. However, at least one presidential powers expert said the move is likely to be challenged in the courts.

The officials said the issue is still under review in the Pentagon, but even as any legal review goes on, the administration's intent is to have troops detain migrants at the border.

The land transfer is the farthest the Trump administration has gone yet to use the military for border security. Up until now, the military's involvement has been limited largely to helping build or fortify sections of the border wall and operating deportation flights to send migrants back to their home countries or, in some cases, to the detention facility in Guantanamo Bay, Cuba. The military also has assisted U.S. Customs and Border Protection agents by conducting surveillance or performing backup administrative tasks.

The corridor, known as the Roosevelt Reservation, is a 60-foot-wide federal buffer zone that ribbons along the border from New Mexico to California, except where it encounters tribal or privately owned land. It had been run by the Interior Department until Trump directed control be transferred to the Defense Department in a presidential memo released Friday night.

For the next 45 days, the Defense Department will test taking control of a section of the Roosevelt Reservation in New Mexico, east of Fort Huachuca, which is an Army installation in Arizona, one of the U.S. officials said. During that period, the Army will put up additional fencing and signs warning people not to trespass.

People not authorized to be in that area could be arrested by the Army's security forces, the officials said, who spoke on condition of anonymity to provide details not yet made public.

Any migrants in the country illegally who are detained by military personnel on those lands would be turned over to local civilian law enforcement agencies, the officials said.

It was not clear if the added land would require the military to deploy additional forces to the border. There are about 7,100 active duty troops under federal control currently assigned to the border and about 4,600 National Guard troops under state control.

Troops are prohibited from conducting civilian law enforcement on U.S. soil under the Posse Comitatus Act. An exception known as the military purpose doctrine allows it in some cases — but would not apply here and would likely be challenged in the courts, said Elizabeth Goitein, an expert on presidential emergency powers at the Brennan Center for Justice.

That's because even though troops would be on land designated as an Army installation, they would have to prove that their primary mission there was not to conduct border security and law enforcement — and the whole point of Trump's order transferring the Roosevelt Reservation to the military's control is to secure the border, she said.

The military purpose doctrine "only applies if the law enforcement aspect is incidental," Goitein said. "Does this (area) have a military purpose that has nothing to do with enforcing customs and security at the border?"