President Donald Trump’s deployment of 5,900 active-duty troops to the southern border ahead of the arrival of large groups of Central American migrants will cost approximately $72 million, the Pentagon said Tuesday.

The estimate spans the “cost to deploy, operate, sustain, and redeploy forces” through Dec. 15, the date by which the entire military mission is scheduled to conclude.

It might be more, the Pentagon cautioned. “The total cost of the operation has yet to be determined and will depend on the total size, duration, and scope of the [Defense Department’s] support to [the Department of Homeland Security],” Pentagon spokesperson Col. Rob Manning said in a statement.

It is the first cost estimate released by the US military since troops began arriving at the border in late October. They were quickly mobilized in what was widely criticized as a political move ahead of the midterm elections as Trump warned of an “invasion,” even though at the time the migrants were hundreds of miles away and most planned to ask for asylum at the border. The troops have spent the ensuing weeks mainly putting up miles of concertina wire.

Earlier Tuesday, US Army North officials said they might “shift some forces to other areas of the border to engineering support missions in California and other areas” in the coming days.

The largest concentration of troops has been in Texas, with 2,800 deployed along the border there and 1,500 in Arizona. Only 1,300 had been deployed in California, according to the latest US Northern Command numbers, even though the majority of migrants with the caravans are heading to the border between California and Mexico.

The deployment of 2,100 National Guard troops, who have been there since April, is expected to cost an additional $138 million, according to a report to Congress obtained by the Associated Press. Some analysts have estimated that Trump’s deployment, which currently is slated to last roughly six weeks, would cost more than $200 million.

When former president Barack Obama deployed 1,200 National Guard troops for 14 months for Operation Phalanx in 2010, it cost $145 million. Former president George W. Bush’s deployment of 6,000 National Guard troops over two years, from 2006 to 2008, ran up a $1.2 billion bill. Government watchdog groups and Pentagon officials criticized both the “absence of a comprehensive strategy” that justified the cost of using US troops without a clearly defined role.

Some Democrats have pressed Defense Secretary Jim Mattis to back up the Pentagon’s decision to send such a large group of active-duty troops, who have not been sent to the border in three decades. National Guard troops would have required the approval of state governors, which, some had signaled during the family separation crisis of last summer, they were not willing to provide.

In a letter on Tuesday, Democratic Sen. Elizabeth Warren and Reps. Jackie Speier and Beto O’Rourke, asked Mattis to answer 13 detailed questions, including how the decision was made, whether the deployment will affect military readiness, and the rules of engagement.

“You have made lethality and readiness for the high-end fight your number one priority as Secretary of Defense,” they wrote. “But we fail to see how sending troops to the border to lay concertina wire in response to a group of migrants serves that end.”

On a visit to soldiers on the border in Texas last week, Mattis said the long-term goals for the military mission were “somewhat to be determined.”