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– The number of migrant family members arrested for illegally entering the United States shot up 38 percent in August, according to statistics released Wednesday, a surge Homeland Security officials characterized as a "crisis."

Border Patrol agents apprehended nearly 13,000 members of "family units" last month, the latest data show, the highest August total ever recorded. The increase followed President Trump's decision to back off the provision of his "zero tolerance" crackdown that separated children from parents in an attempt to deter illegal migration.

Migration numbers typically rebound in August after a summer lull. Overall, the number of foreigners apprehended or deemed "inadmissible" at border crossings rose to 46,560 in August, up from 39,953 in July.

Homeland Security officials said the arrival of so many families was due to court-imposed restrictions limiting the duration children may be detained in immigration jails. The result, officials say, is that parents bring children as a way to win quick release from government custody and avoid deportation.

"The numbers have continued to increase because this is a well-known avenue to arrive in the U.S. and be allowed to stay," said Customs and Border Protection Commissioner Kevin McAleenan, calling the trend "a crisis of significant proportions, from a humanitarian perspective and a security perspective."

Agents working in South Texas described August as a busy month of rafts coming across the Rio Grande and groups so large they had to be loaded onto Border Patrol buses.

Nearly all family members appearing at the border are from the "Northern Triangle" of Central America — Guatemala, Honduras and El Salvador — where homicide rates and grinding poverty have fueled emigration for decades.

What's changed, officials say, is the growing recognition in Central America of what they call "loopholes" in U.S. enforcement.

"Smugglers and traffickers understand our broken immigration laws better than most and know that if a family unit illegally enters the U.S. they are likely to be released into the interior," Department of Homeland Security spokesman Tyler Houlton said.

According to the latest DHS statistics, he added, more than 98 percent of family members from El Salvador, Guatemala, and Honduras who arrived between October 2016 and the end of June are still present in the United States.

The number of migrants under age 18 in U.S. custody also rose last month, and the Department of Health and Human Services said Tuesday it has 12,800 minors in its system of shelters, a record number.