No decision has been made on the numbers of active duty troops that will be sent to the U.S.-Mexico border, but the number could eventually total in the thousands, a defense official told Military Times.

Last week, Defense Secretary Jim Mattis approved sending active duty forces to the U.S.-Mexico border, and initial reports said the department was planning for a size of about 800 active-duty troops to support border agents handling an approaching caravan of thousands of Central American migrants.

But those figures may only reflect an initial response by the Defense Department, which has ordered that the first active-duty forces arrive by Oct. 31.

On Monday morning, the Wall Street Journal reported that the actual figure under consideration by the Trump administration was closer to 5,000. That force would be comprised primarily of military police and engineers, according to the report.

But even in the hours before the report published, defense officials were still working around the assumption that the force would be in the “hundreds,” a defense official said. Three defense officials said on the condition they not be named that no official order had been signed and no final decision had been made on what force size would be deployed. DoD and the Department of Homeland Security will be having a joint press conference later today on the matter.

The total force, which would respond in phases, will likely be “in the thousands,” another defense official said on the condition of anonymity.

The Wall Street Journal reported that the deployment would be focused on Texas, Arizona and California, with mainly military police and engineers from the Army and Marine Corps comprising the force.

About 2,100 National Guardsmen are already deployed throughout southern states in a similar support role. President Donald Trump has insisted the new assistance force of active-duty military members is necessary to prepare for a caravan of several thousand migrants heading from Central America to the United States through Mexico.

But Democrats have accused Trump of militarizing the immigration debate for political reasons, questioning the threat the caravan poses and the usefulness of sending a sizable group of service members to the border with unclear missions.

On Friday, the Departments of Defense and Homeland Security issued a joint statement saying the troops’ mission would involve providing civilian authorities with “planning assistance, engineering support, fixed and rotary wing aviation support to move personnel” as well as medical assistance, temporary housing and protective equipment for border patrol members.

On Sunday, Mattis said the military had already begun moving construction materials to the border as details on the troop deployments were still being finalized.

He acknowledged that work will include discussion of the specific roles military personnel will have, since federal troops are largely prohibited from engaging in domestic law enforcement activities under the Posse Comitatus Act.

The Wall Street Journal reported the mission — to be named Operation Faithful Patriot — is expected to last until at least mid-December.