ABOARD A US MILITARY AIRCRAFT — The military is already moving some construction materials to the border as it finalizes the number of troops and authorities U.S. forces will have in response to a caravan of migrants headed there, Defense Secretary Jim Mattis said Sunday.

Last week, Mattis approved sending additional active duty troops to support about 2,100 National Guard forces already on the border. The active duty forces, who will be under the command and control of U.S. Northern Command, will not have a law enforcement role but instead will be involved in providing engineering support, including delivering temporary barricades and barriers, medical teams and aviation support.

“This is a response to the caravan directed by the president,” Mattis told reporters traveling with him to the Czech Republic.

That extensive planning includes moving jersey barriers down to the border and getting gear and logistics support set in place before thousands of migrants traveling to the border arrive, Mattis said.

“If you look at how we organize for the storms — from Puerto Rico, where you surround the storms, for Texas, for Florida — we surround the storm … so right now, we’re planning it, how we are going to do it.”

It also requires an extensive discussion on what specific roles military personnel will have. Under the Posse Comitatus Act, federal troops are prohibited from engaging in domestic law enforcement activities except in very specific emergency situations.

Those decisions haven’t been finalized yet, Mattis said.

“The order’s being drafted, and some material is moving, construction type material — jersey barriers, that sort of thing,” Mattis said. “I’ll come back to you when we have it finalized, the details of the rules that the troops will be under. … I do not have that at this time.”