Illegal immigration across the southwest border surged 15 percent in July compared to the month before, Homeland Security announced Tuesday, signaling that President Trump’s early success in stemming the flow of people jumping the border has begun to lessen.

The border patrol nabbed 13 percent more illegal immigrants last month than in June, while officers at the ports of entry saw a 23 percent spike in the number of inadmissible aliens showing up and demanding entry. The number of Border Patrol arrests is deemed a yardstick for the overall flow of people.

July usually sees a drop, so the surge signals a change in the usual seasonal patterns.

David Lapan, spokesman for Homeland Security, said they’re still trying to figure out what the trends mean.

“The short answer is we don’t know what all of those causes are,” he told reporters.

The numbers of families and unaccompanied alien children (UAC), which have been particularly troublesome cases, also saw huge spikes. UAC were up 27 percent in Border Patrol statistics, while people traveling together as families surged 46 percent.

Still, even with the rise month-to-month, the numbers are lower than they have been in years.

Customs and Border Protection officers at the ports of entry recorded 6,833 inadmissible aliens trying to enter. The Border Patrol, meanwhile, nabbed 18,198 illegal immigrants who snuck across the border.

The combined number of 25,031, while the highest under Mr. Trump, is still lower than any other month under President Obama dating back until December 2011.

Mr. Obama oversaw a deterioration at the border in his second term, with children and families streaming north from Central America, believing lax enforcement would give them a chance to earn a foothold in the U.S.

The Trump administration has been trying to dig out of that hole with a series of policies including targeting for deportation UAC and families who’ve had their day in immigration court but who are ignoring orders or removal.

Mr. Trump has also asked for money to begin building his border wall, with prototypes to be built later this year.

The president has repeatedly cited progress on the border as one of his major successes early in his tenure. He was so pleased with the progress that he pointed to it as one of the reasons he tapped former Homeland Security Secretary John F. Kelly to become his new chief of staff at the White House.

On a year-to-year basis, the gains are indeed stunning. In July 2016 the Border Patrol nabbed 33,737 illegal immigrants — 85 percent more than this year’s total. Fewer people apprehended likely means fewer people crossing.

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