EXCLUSIVE: Customs and Border Protection (CBP) is shutting down three highway checkpoints in Arizona, Fox News has learned, as the overwhelmed agency struggles to get a grip on a rapidly escalating crisis at the southern border.

The agency is shutting down three checkpoints in Willcox, Arizona, until further notice as personnel are being used instead to deal with the influx of migrants at the border, according to an internal document dated Tuesday seen by Fox News. Shutting down the checkpoints will severely hamper authorities' anti-trafficking efforts, a CBP source told Fox News.

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CBP later said in a statement to Fox: "Tucson Sector has redirected manpower, based on operational needs in response to shifting traffic patterns, resulting in the closure of several tactical checkpoints in southeastern Arizona."

"We expect to re-open these checkpoints as manpower and activity levels dictate," the statement said.

Additionally, a CBP official also told Fox News that the checkpoints had been closed in early January due to shifting migrant patterns in the area, with limited staff left in place for security purposes. The move this week removes the checkpoints entirely in order to free up additional manpower to deal with the shifting migrant patterns.

The announcement comes as the Biden administration is dealing with a spike in child migrants and family units at the border. CBP will release numbers this week and is expected to show there have been approximately 100,000 apprehensions in February.

A senior DHS official confirmed to Fox News that the number of migrant children in custody along the border has tripled in the past two weeks to more than 3,250 and that more than 1,360 have been held for longer than the allowed three days.

Mark Morgan, a former acting CBP commissioner during the Trump administration, said it was not a surprise considering the spike in numbers.

"This is exactly what happened to us in 2019 at the height of the crisis before we were able to get the tools, policies and authorities we needed to address it," he told Fox News. "We were shutting down lanes, we were pulling resources off what I referred to as our national security mission to deal with the humanitarian crisis."

"As you’re looking at daily numbers between 4,500 and 5,000 a day, I knew this was coming," he said.

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Morgan said that closing checkpoints could allow criminal illegal immigrants and drugs to be more easily brought into the U.S.

"When you start closing down interior checkpoints or lanes at ports of entry at the border, you are absolutely increasing the cartel’s ability to smuggle drugs into the country," he said.

He said that closing down checkpoints "means there’s no one there and cars just drive on by."

The administration has ended a number of Trump-era policies implemented to end the 2019 crisis, including the Migrant Protection Protocols (MPP), which kept migrants in Mexico for their hearings, and a number of agreements with Central American countries.

It has also narrowed Immigration and Customs Enforcement (ICE) priorities for arrest and deportation after being blocked from implementing a deportation pause after a Texas lawsuit. Separately, the White House and congressional Democrats are proposing a pathway to citizenship for millions of illegal immigrants as part of an immigration reform bill.

Republicans and others have linked the policy shift to the migrant surge, saying it is incentivizing migrants coming north. The Biden administration has denied there is a crisis, saying instead they are working to end the "cruelty" of the prior administration.

But DHS Secretary Alejandro Mayorkas on Monday emailed staff requesting volunteers for a "Volunteer Force" to help CBP at the border, descrived "overwhelming numbers" of migrants.

"You have likely seen the news about the overwhelming numbers of migrants seeking access to this country along the Southwest Border," he said, in an email obtained by Fox News. "President Biden and I are committed to ensuring our Nation has a safe, orderly, and humane immigration system while continuing to balance all of the other critical DHS missions."

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That request comes as the administration is increasing the number of processing facilities and child migrant facilities to handle the influx. Health and Human Services has said it is considering using a Virginia military base to house child migrants.

On Tuesday, White House press secretary Jen Psaki would not say if there was a crisis at the border but did say, "There is an increase, that there are more children coming across the border than we have facilities for at this point in time."

Fox News' Griff Jenkins contributed to this report.