After senior U.S. military officials told Congress they do not know yet how much the surge of active-duty military forces to the southern border with Mexico and the buildup of infrastructure to house thousands of migrants at Guantanamo might end up costing, two Democratic senators on Friday requested more details from the Pentagon.

Following a Senate Armed Services Committee hearing on Thursday with the commanders of U.S. Northern Command and U.S. Southern Command, Sens. Mazie Hirono, D-Hawaii and Elizabeth Warren, D-Mass., sent a letter to Defense Secretary Pete Hegseth requesting specific details by Feb. 27 regarding the costs, impact on readiness and impact on morale that would result from the new missions along the border and at Guantanamo.

“We are concerned about the Department of Defense’s (DoD) immigration-related operations at the southern border and at Naval Station Guantanamo Bay — including the implications of these operations for the military’s budget, readiness, and morale,” the senators said in the lengthy letter requesting specific details from the Pentagon.

“DoD’s support for the Department of Homeland Security (DHS) has been expensive for American taxpayers, with some DoD expenses costing over three times more than when DHS performs the same function, while also posing “an unacceptable risk” to units’ readiness,” they wrote.

“In all, the Trump administration is militarizing the country’s immigration enforcement system in an apparent attempt to signal toughness,” they added. ” But this political stunt will come at a high cost; it risks diverting DoD’s resources away from its vital mission in ways that compromise our national security.”

At Thursday’s hearing, Gen. Gregory Guillot, the commander of NORTHCOM, and Adm. Alvin Holsey, the commander of SOUTHCOM, were asked about the potential impact on training and readiness for the forces sent to the border and what the financial costs were for the surge as well as the construction of migrant housing at the naval base at Guantanamo, Cuba.

Guillot told the committee that the number of active-duty forces now on the Southern border with Mexico had risen to 5,000 and said he expected the number to continue to rise.

Included in the 5,000 are the additional troops deployed to join the 2,500 federalized National Guardsmen and reservists who had been serving as part of a border mission established by the Trump administration in 2018.

Meanwhile, the most recent deployment to the border took place this week as 500 Army soldiers from the 10th Mountain Division from Fort Drum, New York, arrived at Fort Huachuca, Arizona, to establish a coordinating headquarters for the border mission. An additional 1,000 troops from Fort Bragg, North Carolina, are awaiting orders to deploy to the border in support of that unit, according to a U.S. official.

Asked about potential costs for the operation on the border, Guillot explained that his command has “not appropriated funds for the Southwest border, and we’ve never had reprogramming or pass through funding. This is all done through the Department Comptroller and the services.”

The NORTHCOM commander also told the committee that he did not believe that the cycling of troops to the southern border would impact any scheduled training exercises being held by his command though he noted there might be a reduction in the one day of weekly training provided to deployed forces conducting jobs outside of their normal specialized roles.

“I think that specifically in the helicopters and in our intel specialties that are conducting missions on the southern border, those are exactly in line with their trained specialty,” said Guillot. “However, there are, I certainly recognize there are areas where they are cross-trained and they are not getting immediate benefit to their primary specialty in about half of those roles.”

At the naval base at Guantanamo, 500 Marines have been deployed to erect tents and facilities that could potentially house as many as 30,000 migrants while they await processing to be returned to their home countries.

Adm. Holsey described the new construction at Guantanamo as a “phased” approach currently targeting housing for up to 2,500 migrants and said the potential expansion to 30,000 migrants would depend on the Department of Homeland Security’s flow of migrants to the base.

“We’re going to build it up and as we understand how large we’ll get, we’ll have a better focus on the price,” said Holey when asked how much those operations could end up costing.

He added that the Department of Homeland Security is providing security for the detained migrants and noted that any current military costs in expanding facilities at the base are coming from the military services’ existing budgets.

According to Holsey, there are currently 93 deported migrants currently housed at Guantanamo, with 63 of them being housed at the main prison facility that used to house hundreds of enemy combatants seized during the War on Terror. The remainder are all being housed in the newly expanded housing that falls under the base’s Migrant Operations Center.