(CNN) - Nearly 400 migrants burrowed under a fence on the US-Mexico border earlier this week and crossed into the United States, four sources familiar with the matter told CNN.
US Border Patrol officers stationed in the Yuma, Arizona, sector took about 375 migrants into custody on Monday after they had made it into the United States, one US official said, calling it an unusually large apprehension. It was not immediately clear if the migrants voluntarily surrendered to Border Patrol officials or if they were caught after attempting to evade authorities.
The "vast majority" of the group were family members arriving from Guatemala, said National Border Patrol Council President Brandon Judd, who was in contact with a Border Patrol agent involved in the incident in Yuma.
"It's the largest I've ever heard of," said Judd when asked if this was a significant number for a single group.
There are often groups of 20 to 30 people, sometimes as large as 100, apprehended in the Yuma area, according to a Customs and Border Protection official.
A Customs and Border Protection agency spokesperson declined to comment.
The incident comes as President Donald Trump continues to demand funding for new barrier construction on the US-Mexico border as the government shutdown nears its fourth week, saying there is a "crisis" at the border that can only be solved with the construction of new border walls or fencing.
While Border Patrol officials have discovered sophisticated tunnels used to move people or drugs under existing border fencing, one US official familiar with the incident said the migrants involved in Monday's crossing had simply dug a hole under the border fence and slipped underneath. A Customs and Border Protection official confirmed that "a large group" of migrants "breached the border wall."
"They dug under the fence," said Judd. "Unlike most areas along the border, Yuma has some very sandy areas that is easy to dig in."
The migrants were taken into custody about 10 miles east of the San Luis Port of Entry, along a stretch of the US-Mexico border lined with older fencing -- steel panels welded onto posts that previously served as vehicle barriers, with little foundation to speak of.
Some fencing in the Yuma area is expected to be replaced with new fencing paid for with funds appropriated by Congress last year. CBP has not confirmed exactly where the new fencing will be placed.
In November, a group of around 80 migrants from Guatemala -- primarily families -- were apprehended by Border Patrol after climbing over the legacy landing mat border wall east of the San Luis Port of Entry. Hours later, another group of around 80 people entered the United States by digging a shallow hole underneath the same portion of wall, according to CBP.
"This old portion of wall lacks the improved concrete footer that new wall prototypes have which prevents easy digging underneath," CBP said in a press release at the time.
Monday's group of migrants was the first of at least two large groups to arrive at the US border this week. Nearly 250 migrants -- mostly Central American migrants, including families and small children -- were also detained by Border Patrol agents on Wednesday after surrendering at a Border Patrol outpost on the New Mexican border.
However, it used to be much higher. In the mid-2000s, more than 100,000 migrants were apprehended annually in the Yuma region.
The Trump administration has pointed to a reduction in border crossings from their all-time high in Yuma sector to make the case that walls work.
Border Patrol apprehensions of migrants across the US-Mexico border have increased in recent months, spurred on by a surge in families seeking asylum at the US border.
But border security experts have questioned whether Trump's demand for nearly $6 billion in funding for a new steel slat barrier along hundreds of miles of the border is the best use of resources to stem illegal immigration into the United States. Questions also remain about whether a wall would alleviate the humanitarian situation driven by an uptick in asylum seekers presenting themselves at the US border.
Some border security experts have instead called for improving security at ports of entry and adding new technology along the border to detect and deter illegal crossings.
Even as statistics show that nearly 90% of drugs that come through the southern border are concealed and pass through legal US ports of entry, Trump has insisted without evidence that a border wall will help drastically reduce the flow of drugs and crime in the United States.
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