Department of Homeland Security Secretary Kirstjen Nielsen took a hard line Thursday against the thousands of Central Americans headed toward the United States — but she acknowledged that legal loopholes make it hard for her to back up those words.

Appearing on Fox News’ “The Story with Martha MacCallum,” Nielsen also confirmed that criminals and people who pose potential security threats are traveling among the crowd.

Nielsen (pictured above left) insisted the caravan should not be allowed to arrive here. It is conservatively estimated to include 5,000 people but could be twice that size. The long line of migrant marchers is several weeks from reaching the U.S. border.

“This caravan cannot come to the United States,” Nielsen said. “They will not be allowed in. They will not be allowed to stay.”

Nielsen, who was in Yuma, Arizona, to tour a section of the new border wall, said some 30 percent of women and girls and 17 percent of men experience sexual assault when they make that trip. She said about 70 percent of all migrants experience violence in one form or another.

Many people are making the dangerous journey based on a misunderstanding of how U.S. asylum law works, Nielsen said.

“If you are coming here to seek family reunification, that is not asylum,” she said. “You cannot stay. If you are coming here to seek a job, you cannot stay.”

“If you are coming here to seek family reunification, that is not asylum. You cannot stay. If you are coming here to seek a job, you cannot stay.”

But when pressed on how the United States would stop the Central Americans, Nielsen offered little in the way of specifics. She mentioned that the Department of Defense (DOD) is making available about 800 servicemen to bolster 1,600 National Guard personnel. But those soldiers will be limited to support functions only.

“Everything’s on the table,” she said. “We’re talking about every possible way in which we can adjust the flows.”

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The same obstacles that frustrate immigration enforcement generally will hinder efforts to stop the caravan, Nielsen said. Under U.S. law, Mexicans crossing the southwest border illegally can be sent back within hours. Because of different rules in effect for non-Mexicans — particularly non-Mexican children — the process takes much longer.

Nielsen said it takes as long as 3,000 days to deport someone to Central America who is not held in detention. She said U.S. Border Patrol officers apprehend more families in three days than the United States can deport in a year.

Under a court interpretation of an agreement that settled a 1980s lawsuit, the United States cannot detain adults and children traveling together for longer than 20 days. This means they must be released into the interior of the country to await their immigration court proceedings.

“It’s sort of like having both my hands tied behind my back,” Nielsen said. “I still have the same mission, but Congress has prohibited us from executing it.”

“This caravan cannot come to the United States. They will not be allowed in. They will not be allowed to stay.”

Asked about President Donald Trump’s contention that Middle Easterners and criminals are in the caravan, Nielsen told MacCallum that she has no way of knowing for sure who is traveling with the throng since they pushed past two international boundaries without following proper procedures.

“What we do know is that terrorists have highlighted for many years the loopholes in our border security,” she said. “What we do know is there are criminals as part of this flow. We do know there are gang members as part of this flow.”

As for potential terrorists, Nielsen said U.S. Border Patrol officers every year apprehend about 3,000 “special-interest aliens” — people from countries associated with or compromised by terrorism.

The Trump administration earlier this year responded to the 20-day limit on detention of children by placing them in the custody of the Department of Health and Human Services (HHS) while bringing criminal charges against the parents or those claiming to be parents.

Outrage over family separation forced a reversal. Today, Nielsen said, a few hundred children remain separated, while their parents have been deported to their home countries.

Related: Hill Immigration Republicans Wary of Approaching Migrant Caravan

“And the parents now, three different times, have said they do not want to be reunited,” she said. “So they’re not reunited.”

Nielsen said smugglers know that illegal immigrants with children have an exponentially better chance of being able to stay. She said that is why so many more Central Americans are bringing their children — even other people’s children.

Nielsen said she even has received reports from governments in the region that kidnappings have occurred as part of efforts to exploit U.S. immigration law.

“Fraud is through the roof,” she said. “Those of us who have visited in the region are told, absolutely, that it is advertised as a way to get into the United States — to take a child and bring them here.”

Nielsen reiterated that U.S. officials remain focused on the approaching mass of migrants.

“At the end of the day, we want to make sure that what we saw in Guatemala, between the Guatemalan and Mexican border, we do not want to see here,” she said. “So we’re bringing everyone we can to protect the ports of entry and all of the people who work at them.”