Federal authorities apprehended 40,513 people who either crossed the southwest border from Mexico into the United States or directly turned themselves in at border stations during the last month of 2017.

The December total marks the eighth consecutive increase since the number plummeted to 15,766 in April — but it was the highest since authorities caught 42,462 in January 2017, a month during which Trump was president for only 12 days.

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Experts estimate that for every apprehension, one illegal immigrant makes it through to the interior of the United States.

Tyler Houlton, acting press secretary at the Department of Homeland Security, noted that the overall border crossings for 2017 — 341,084 — represent a 40 percent decrease from the prior year.

“The final border apprehension numbers of 2017, specifically at the southern border, undeniably prove the effectiveness of President Trump’s commitment to securing our borders,” he said in a statement.

A big chunk of the recent increase has come from unaccompanied children, mostly from Central America, and adults traveling with children. Without that group, apprehensions actually declined from 18,077 in November to 16,792 in December.

Under a ruling by a federal judge in California, the U.S. Border Patrol cannot hold children for more than 72 hours. After that, they must be transferred to the Office of Refugee Resettlement, which places the youths with sponsors in the United States.

Experts who have studied the issue have concluded that the children often end up with illegal immigrants and frequently disappear into U.S. society while waiting for immigration court dates that they skip in high numbers.

Houlton called for Congress to address the issue.

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“Current loopholes in our immigration laws have created an incentive for illegal immigrants who knowingly exploit these same loopholes to take advantage of our generosity,” he said in the statement. “The secretary will require fixes to these loopholes as part of any immigration package negotiated today at the White House.”

Jessica Vaughan, director of policy studies for the Center for Immigration Studies, said the judicial ruling — upheld by the San Francisco-based 9th U.S. Circuit Court of Appeals — has tied the hands of immigration enforcement officers. The judge interpreted a 1997 settlement agreement as prohibiting the long-term detention of minors accused of violating immigration laws.

Congress could pass a law superseding that consent decree, Vaughan said.

Vaughan pointed to the steadily rising number of border crossings as evidence for the need for Congress to back Trump’s rhetoric with concrete actions to beef up border security.

“It looks very much like the smugglers have figured out where the vulnerabilities are … The initial effects of Trump’s new approach, which really scared people from coming, [have] worn off somewhat.”

“What this says to me is that it’s clear there’s only so much the president can do without more money from Congress and some tweaks of the laws,” she said. “It looks very much like the smugglers have figured out where the vulnerabilities are … The initial effects of Trump’s new approach, which really scared people from coming, [have] worn off somewhat.”

Vaughan added that the administration cannot simply deploy more resources along the border. She said interior enforcement is important, too — including a commitment to removing the magnets that draw illegal migrants in the first place.

Related: Trump: U.S. Can’t Have Border Security Without the Wall

“Even though there have been real tangible improvements at the border, there hasn’t been an increase in enforcement at workplaces,” she said.

Matthew O’Brien, director of research at the Federation for American Immigration Reform (FAIR), said rhetoric works both ways. On the one hand, he said, foreigners responded to the president’s tough talk early in his term.

But on the other hand, he added, they also have responded to talk in Washington of extending permanent legal residency to illegal immigrants enrolled in the soon-to-expire Deferred Action for Childhood Arrivals (DACA) program.

Related: Noncitizens Make Up One in Five Federal Prisoners, Report Shows

“When you say you’re gonna reward bad behavior, you’re going to see more bad behavior,” O’Brien said.

That talk has heated up in recent weeks, culminating Tuesday with a meeting at the White House with top lawmakers, in which Trump spoke favorably about a possible deal on the Development, Relief, and Education for Alien Minors (DREAM) Act.

Top leaders have been talking about a “fix” for DACA for months.

“That [rising border crossings] seemed to coincide with talk of amnesty, talk of the DREAM Act,” said Spencer Raley, a research associate for FAIR.

PoliZette senior reporter Brendan Kirby can be reached at [email protected]. Follow him on Twitter.