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 doubled down on his criticism of Central American countries whose citizens are trekking toward the U.S. in search of asylum, shining a spotlight on the issue on Monday as he headed to Houston for a campaign rally.

Trump was asked as he departed the White House whether he has any evidence to support an assertion in an early morning tweet that there are "criminals and unknown Middle Easterners" in the caravan. Instead, he invited reporters to investigate for themselves.

"You know what you should do, Jon? Go into the middle of the caravan, take your cameras and search," Trump told ABC's Jonathan Karl.

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"You’re going to find MS-13, you’re going to find Middle Eastern, you’re going to find everything," the president added. "And guess what? We’re not allowing them in our country. We want safety."

Trump in recent days has seized on the group of thousands of Central American migrants making their way toward the U.S. border to stoke fear about the potential influx of illegal immigration, reigniting a hot-button campaign issue with the Nov. 6 midterm elections just two weeks away.

CNN reported earlier Monday that the group had grown to more than 7,500 people, and is expected to reach the U.S. border in about a week.

"It's a horrible thing, and it’s a lot bigger than 5,000 people," Trump said outside the White House Monday.

The president has been fiercely critical of Honduras, El Salvador and Guatemala for not stopping their citizens from fleeing, and he reiterated his pledge to cut off or reduce aid to those countries as punishment.

"We give them foreign aid, and they did nothing for us. They did nothing for us," Trump told reporters. "We give them tremendous amounts of money — you know what it is you cover it all the time — hundreds of millions of dollars. They, like a lot of others, do nothing for our country."

Guatemala received $249 million in U.S. aid in 2017, while Honduras received $175 million and El Salvador received $115 million, according to the United States Agency for International Development.

Experts have said there are human rights laws that prohibit countries from stopping citizens from leaving their borders.

The White House did not respond to questions from The Hill about how Trump would go about cutting aid. Congress is not scheduled to reconvene until after the midterm elections.