Hundreds of miles up the road in southern Mexico, the first caravan of some 4,000 migrants resumed its advance, still at least 1,000 miles or farther from their goal of reaching the United States as the Pentagon announced it would send 5,200 active-duty troops to "harden" the U.S. -Mexico border. "We will continue to adjust the number and inform you of those", O'Shaughnessy told reporters.
Guatemala's Interior Ministry said the second group had wounded Guatemalan police and used children as human shields, and Guatemalan firefighters confirmed that a 26-year-old Honduran had been killed from a blow to the head. All of the active-duty troops are scheduled to be deployed by the end of the week, but some are already in the Rio Grande Valley.
O'Shaughnessy said some soldiers would be armed although it was unclear who, beyond USA military police, might need those weapons.
The move represents a massive military build-up along the border, where some 2,000 National Guardsmen are already working to provide help to overwhelmed authorities.
His warning comes as the USA military is reportedly preparing to send 5,000 troops to the border, a significant increase over the initial estimates of 800 reported last week.
U.S. Air Force General Terrance J. Only about 200 in that smaller group made it to the border.
O'Shaughnessy declined to comment on intelligence about the caravan when asked whether there were terrorists among the migrants.
The Interior Department said in a statement that two Hondurans ages 17 and 22 were arrested Monday when one of them tried to shoot at police in the town of Ignacio Zaragoza, near the Hidalgo border crossing.
The aviation units are among ten listed in the planning documents which are slated to be in place by November 2 and conclude their mission on December 15.
"The president has made it clear that border security is national security", O'Shaughnessy said, as he detailed a much larger deployment that the 800 to 1,000 troops predicted by US officials last week.
His experience with the caravan had taught him one thing, he said: "The next time, I'm going to need a 'coyote, '" or smuggler.
Military police units are also being deployed.
McAleenan said the aim of the operation was to deter migrants from crossing illegally, but he conceded his officers were overwhelmed by a surge of asylum seekers at border crossings.
That's because the military is bound by the Posse Comitatus Act, a 19th- century federal law that restricts participation in law enforcement activities.
There are already 2,100 National Guard troops at the border, sent after a previous Trump request in April.
The military operation drew quick criticism.
Mr Trump, who has seized upon the Central American migrant caravan in campaign rallies ahead of the vote, said on Twitter that the military would be waiting for the procession - suggesting a far more direct role in confronting the migrants than U.S. defence officials have previously suggested.
The migrant caravan has been moving slowly north from Central America and its numbers have been dwindling.
O'Shaughnessy said that the U.S.is "working closely to understand the nature and makeup of this caravan".
"The migrant caravan is full of women and children fleeing violence, poverty, and government repression", Markey tweeted Monday. On Monday he said he wants build tent cities to house asylum seekers, and on Tuesday he floated the possibility of ending the constitutional right to USA citizenship for babies born in the country to non-citizens.