The number of children surging across the border illegally quadrupled in May compared with a year ago at the same time, and the number of people traveling as families is up sixfold, according to Homeland Security statistics released Wednesday.

Border Patrol agents caught 6,405 children trying to jump the border last month and nabbed another 9,485 “family units.” A year ago in May, those numbers were 1,493 unaccompanied alien children (UAC) and 1,577 family units.

Officers at the ports of entry encountered an additional 830 UAC and 4,718 family members traveling together. That number included people who planned to seek asylum.

The number of people caught is considered a rough estimate for the overall flow of people crossing, so the surge suggests that more people are making it across the border undetected.

When all immigrants are included, overall illegal crossings are 2½ times higher than last year at this time.

“No one expects to reverse years of political inaction overnight or in a month,” said Homeland Security press secretary Tyler Q. Houlton.

He said part of the responsibility for solving the problem rests with Congress.

“Smugglers, human traffickers, and nefarious actors know our loopholes well and accordingly exploit them. The refusal by members of Congress to close catch-and-release loopholes have prevented the administration from controlling the border,” he said.

It may even have been Congress that helped spark the surge.

The Senate in January began a debate on how to grant citizenship rights to millions of illegal immigrants. President Trump joined the debate with his own legalization plan.

Two months later, the overall number of illegal immigrants stopped at the border topped 50,000 for the first time under Mr. Trump.

The journey north can take weeks or months, particularly for the Central Americans who now make up a large part of the flow, so it’s likely many of those new arrivals made their decision during the legalization debate.

Mr. Trump’s top deportation official said this week that talk of legalization entices new waves of illegal immigration.

“It certainly increases illegal crossings. That’s been proven for the last three decades I’ve been around,” said Thomas D. Homan, chief of U.S. Immigration and Customs Enforcement.

Many of the new arrivals say they are fleeing violence back home and plan to seek asylum in the U.S.

The administration is trying to funnel them to official border crossings.

But the big jump last month came in people who didn’t show up at ports of entry but sneaked into the U.S. and were nabbed by Border Patrol agents.

Hoping to get a handle on the matter, the Trump administration early last month announced a zero-tolerance policy for border crossers, with criminal charges filed against those who enter illegally.

But given the length of time it takes to make the journey north, people caught in May likely began their trip well before that policy, so it’s not clear whether it is having any deterrent effect.

“These numbers show that while the Trump administration is restoring the rule of law, it will take a sustained effort and continuous commitment of resources over many months to disrupt cartels, smugglers and nefarious actors,” Mr. Houlton said.

Mr. Trump’s critics say the zero-tolerance policy is cruel because parents who jump the border with their families are sent to jail and separated from their children.

Children’s advocates say that risks imposing lasting trauma on the children. Chris Padula, executive director of the Center for Youth Wellness, said it can result is what health officials call “toxic stress.”

“Children who are separated from their parents, held in detention centers and deprived of the support of safe and trusted adults may experience serious medical effects over the course of their entire lives,” he said.

Entering the U.S. illegally is a misdemeanor and re-entering after being deported is a felony — though previous administrations, for the most part, declined to pursue criminal cases and instead treated it as a deportation issue.

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