U.S. Sen. Elizabeth Warren, D-Massachusetts, continued to raise questions this week about the Trump administration’s decision to send thousands of American troops to the U.S.-Mexico border -- an operation which officials recently extended.

Warren, who sits on the Senate Armed Services Military Personnel subcommittee, called Acting Defense Secretary Patrick Shanahan Monday to respond to a series of questions on the administration’s Jan. 14 move to continue the deployment through the end of September.

The senator, in a letter sent in conjunction with U.S. Rep. Jackie Speier, D-California, specifically asked Shanahan to explain why the Pentagon decided to extend the deployment, what activities active duty military forces will conduct at the border and why they -- and not Department of Homeland Security staff or National Guard members -- have been chosen for the mission.

The lawmakers further called on the Pentagon to, among other things, provide details by Feb. 11 on: the cost of the operation and its extended deployment; whether it has prepared plans for constructing a U.S.-Mexico border wall; and if the agency’s actions “have had a demonstrable positive impact on border security.”

Arguing that the initial deployment “represented a virtually unprecedented use of active-duty military forces at the border with little or no justification or rationale,” the Democrats questioned whether President Donald Trump may have “fueled unprecedented uncertainty and anxiety inside the Pentagon.”

They further voiced concerns over reports that Trump may declare a national emergency at the U.S.-Mexico border to construct his proposed border wall.

Warren, who is eying a 2020 presidential run, previously pressed Pentagon officials in November for information on the administration’s decision to send 5,000 troops to the southern border ahead of the midterm election.

Defense officials responded to the senator’s initial request for details on the deployment in a Dec. 20 letter.

Undersecretary of Defense John Rood noted that the agency provided support to U.S. Customs and Border Protection “pursuant to presidential direction.”

“The military’s presence and support increases the effectiveness of CBP’s border security operations, frees up border patrol agents to conduct law enforcement duties and enhances situational awareness to stem the tide of illegal activity along the southern border of the United States,” he wrote. “The Department of Defense has a long history of supporting the Department of Homeland Security and CBP in their mission to secure the U.S. border.”

The Pentagon announced earlier this month that it had approved a DHS request for assistance through Sept. 30. Defense officials, the agency noted, would transition their support at the southern border “from hardening ports of entry to mobile surveillance and detection, as well as concertina wire emplacement between ports of entry.”