The top Democrat on the House Armed Services Committee is questioning President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE on the legality of expanding authorities for U.S. troops at the southern border.

In a letter to Trump released Monday, Rep. Adam Smith David (Adam) Adam Smith40 groups call on House panel to investigate Pentagon's use of coronavirus funds When '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 MORE (D-Wash.), the presumptive chairman of the committee come January, said Trump’s decision to allow troops to use force, conduct crowd control and search and temporarily detain people “is yet another unnecessary step towards the militarization of the southern border.”

“Furthermore, the activities described in the memorandum may be a violation of existing federal laws, such as the Posse Comitatus Act (18 USC 1385) or limitations on troops’ participation [in] certain law enforcement activities (10 USC 275),” Smith wrote.

Posse Comitatus is an 1878 law that prohibits the use of the military for domestic law enforcement in most cases.

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At issue is a memo the White House signed last week that allows U.S. military personnel to “perform those military protective activities that the Secretary of Defense determines are reasonably necessary to ensure the protection of federal personnel, including a show or use of force (including lethal force, where necessary), crowd control, temporary detention, and cursory search.”

“Department of Defense personnel shall not, without further direction from the President, conduct traditional civilian law enforcement activities, such as arrest, search, and seizure in connection with the enforcement of the laws,” the memo adds.

On Wednesday, Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE insisted troops are not authorized to engage in law enforcement activities.

“The one point I want to make again is we are not doing law enforcement,” Mattis told reporters at the Pentagon. “We do not have arrest authority. Now the governors could give their [National Guard] troops arrest authority. I don't think they've done that. But there is no arrest authority under Posse Comitatus for the U.S. federal troops. You know, that can be done, but it has to be done in accordance with the law, and that has not been done nor has it been anticipated.”

About 5,800 U.S. troops were ordered to deploy to the U.S.-Mexico border in late October as Trump focused on a caravan of Central American migrants traveling through Mexico ahead of the midterm elections.

Thousands of migrants from the caravan are currently in Tijuana. Tensions flared at the border Sunday as Customs and Border Protection shut down the busy San Ysidro port of entry and some migrants who broke away from the larger group were sprayed with tear gas when they tried to cross a border fence.

The order for the military deployment, which has largely seen troops put up barbed wire, is scheduled to end Dec. 15.

In addition to legality, Smith in his letter raised concerns about the White House’s lack of communication with Congress on the military operation on the border.

“Congress has frequently heard about decisions made or actions taken through the press rather than hearing directly from your administration,” he wrote. “This includes this most recent memorandum issued the week of Thanksgiving.”

Smith asked Trump for a detailed justification for the memo, an assessment of the capability of border patrol that warrants the use of active-duty military personnel in the activities described in the memo and a legal opinion on how the activities authorized by the memo comply with the law.

He also asked for a list of equipment and weapons military personnel are allowed to carry, details of training military personnel will have, the cost of the mission, what specific mission each unit deployed to the border has and what facilities the military would use to temporarily detain asylum seekers.

“I hope that you will reconsider this recent decision,” Smith concluded, “and de-escalate the situation by removing active-duty troops from the southern border.”