Top national security House Democrats are demanding more information from the Pentagon and Department of Homeland Security (DHS) on last week’s deployment of National Guard troops to the U.S.-Mexican border.

“We require a clearer explanation of the impetus for this approach at a time when border crossings are at a 40-year low,” the lawmakers write in a Wednesday letter to Defense Secretary James Mattis and Homeland Security Secretary Kirstjen Nielsen.

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“As you are undoubtedly aware, border security has been a rigorously discussed topic over the past year. We are surprised and puzzled as to why neither of you nor other members of the Administration mentioned a possible deployment of the National Guard to the southern border before,” reads the letter, signed by House Armed Services Committee ranking member Adam Smith David (Adam) Adam SmithWhen 'Buy American' and common sense collide Overnight Defense: Marine Corps brushes off criticism of Marines' appearance in GOP convention video | US troops injured in collision with Russian vehicle in Syria | Dems ask for probe of Vindman retaliation allegations Democrats press Pentagon watchdog to probe allegations of retaliation against Vindman brothers MORE (D-Wash.), Homeland Security Committee ranking member Bennie Thompson Bennie Gordon ThompsonHouse panel pans ICE detention medical care, oversight Senate to hold nomination hearing for Wolf next week Hillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers MORE (D-Miss.), Judiciary Committee ranking member Jerrold Nadler (D-N.Y.), and Oversight and Government Reform Committee ranking member Elijah Cummings (D-Md.).

Mattis last week issued a memo authorizing as many as 4,000 National Guard troops to bolster DHS's border security efforts as part of President Trump’s announcement of measures to further protect the southern border until his long-promised wall is built.

In a House Armed Services Committee hearing Thursday, Mattis said that border patrol agents typically see an uptick in crossings this time of year and the Guard deployment is “an anticipatory backing-up so that the Customs and Border Patrol can put more troops in the field.”

Mattis also insisted that the move “right now . . . is not a long-term deployment.”

“The President says if the wall gets built, that is one thing. Secretary Nielsen says she needs this reinforcement, that is a second point,” Mattis told lawmakers. “This is not a long-term strategy, this is a buying time effort.”

Mattis would not comment on the overall strategy to send troops to the border, saying “that’s rightly under the Department of Homeland Security.”

But he added: “I have read enough of it to know what I needed to do to put these ... up to 4,000 down there until the 1st of October in a non-law enforcement, no contact with the migrants position to support DHS.

Pentagon officials have not said whether the effort will be paid for with Department of Defense (DOD) dollars, or how the department will support the plan, intended to address a "surge of illegal activity" along the border, according to administration officials.

“Given that DHS and DOD have previous experience with this type of deployment, we know that using the National Guard in a support role to the U.S. Border Patrol is a costly endeavor,” the lawmakers wrote, referring to similar Guard deployments under President George W. Bush in 2006 and President Obama in 2010.

Bush’s Operation Jump Start, in effect from 2006 to 2008, “cost $1.2 billion for the deployment of 6,000 National Guard personnel in California, Arizona, New Mexico and Texas,” according to the letter.

The lawmakers ask Mattis and Nielsen to say when DHS and DOD completed an assessment that identified the Guard as a necessity along the southwest border, and the factors used to determine the cost of the endeavor.

They also want more details on the total number of guardsmen to be sent, who will govern them, what role they will play along the border, the estimated cost of the deployment and where the money will come from.

So far, administration officials have said the troops have been barred from interacting with migrants or any people that Border Patrol agents detain, and would not be armed unless necessary for self-protection.

“A hastily and poorly designed deployment could have significant negative impacts on our national security, border security, and on the communities located along the U.S.-Mexico border,” the lawmakers add.

The number of Guardsmen each state will send is still up in the air. As of this week, Arizona will send 338 of its Guardsmen, while Texas will send at least 1,000.

California Gov. Jerry Brown (D) said Wednesday that his state will accept federal funding to add 400 National Guard troops.