Updated at 10:20 p.m. ET
Defense Secretary Jim Mattis is expected to sign an order sending at least 800 U.S. troops to the U.S.-Mexico border as a caravan of thousands of migrants heads north from Central America.
In another possible response to the approaching caravan, news organizations including The Washington Post and Politico quote administration sources as saying the White House is considering an executive order that would close the southern border to migrants, including asylum seekers.
The Department of Homeland Security requested the troops, NPR's Tom Bowman reports, after President Trump said he is intent on stopping any of the migrants from entering the U.S.
The support troops will likely include engineers and military police officers. Citing officials at the Department of Defense, Bowman notes that the troops are expected to serve in support of Customs and Border Patrol and to have no law enforcement duties of their own.
Active-duty military are barred by federal law from participating in civilian law enforcement. The security and support operation, which is likely to include a mix of active-duty and Army Reserve troops, is expected to be running by Tuesday.
This week, members of the caravan made their way through a border crossing in southern Mexico, crossing from Guatemala into Ciudad Hidalgo in the state of Chiapas near the Pacific coast — roughly 1,100 miles from the U.S. border, via the most direct route to Brownsville, Texas.
Immigration and customs agents in the area have already been bolstered by some 2,100 National Guard troops, as the Trump administration has made it a priority to prevent people from illegally entering the U.S.
On Thursday, the president tweeted a word of warning to the migrants making their way north.
"To those in the Caravan, turnaround, we are not letting people into the United States illegally," he said. "Go back to your Country and if you want, apply for citizenship like millions of others are doing!"
It is legal to apply for asylum at a U.S. port of entry, as many of the members of the caravan say they intend to do — and as many members of another caravan did earlier this year. And it is legal to apply for asylum within one year of arriving in the U.S.
It is also impossible to, as Trump suggests, obtain citizenship without first living in the U.S. for several years. Being a permanent resident of the country is one of the steps necessary for naturalization.
As it has headed north, the migrant caravan has grown to include thousands of people. Many of them are from Honduras, where the movement originated; others have come from neighboring countries, such as El Salvador, Guatemala and Nicaragua.
One member of the caravan, 34-year-old Gisele Vasquez, told NPR's Carrie Kahn that she fled Honduras after her husband was murdered by gang members. Without the money to pay a smuggler, Vasquez said she waited to join a caravan large enough to appear safe — and joined this one, together with her sister and their seven children, after hearing about it through social media.
Trump has frequently criticized both the caravan and the countries whose citizens joined it, pledging earlier this week to "begin cutting off, or substantially reducing" U.S. foreign aid to three of the Central American countries.
At campaign events and in tweets, the president has blamed Democrats for making it "tough for us to stop people at the Border."
But, Trump tweeted Thursday, "they will be stopped!"
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