A screengrab from a Yuma Sector Border Patrol video shows two migrants crawling through a hole dug under the U.S.-Mexico border wall at the San Luis port of entry near Yuma, Ariz., in November 2018. Yuma Sector Border Patrol via AP

The migrants turned themselves into authorities and are believed to be the biggest group taken into U.S. custody at one time.

The largest single group of asylum-seekers to be taken into custody in the U.S. reportedly tunneled beneath a section of the border wall outside Yuma, Arizona.

Border Patrol and its parent agency, Customs and Border Protection, this week said that 376 people crossed beneath the barrier, most by traveling through seven tunnels – each only a few feet long – that were dug by smugglers.

"They dug under the fence," National Border Patrol Council President Brandon Judd told CNN. "Unlike most areas along the border, Yuma has some very sandy areas that is easy to dig in."

The group turned itself into Customs and Border Protection agents, according to ABC News, which first reported the incident. It included 30 so-called unaccompanied minors, or children traveling without their parents.

People seeking to cross into the U.S. illegally often travel in groups to avoid being preyed on by smugglers or others during the journey to the U.S.-Mexico border. Some 400 miles east in Lordsburg, New Mexico, for example, two-dozen large groups – consisting of at least 100 people – have crossed the border since Oct. 1, according to CBP.

The record-large group that entered the U.S. on Monday, meanwhile, made the crossing in a particularly rural area, a 26-mile stretch of an older section of barrier that Border Patrol said was being monitored by only three agents.

"In my 30 years with the Border Patrol, I have not been part of arresting a group of 376 people," Chief Patrol Agent Anthony Porvaznik, who oversees CBP's Yuma Sector, told ABC News. "That's really unheard of."

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The number of people being taken into custody after illegally crossing the border has fluctuated in recent years between 400,000 and 570,000 people. In fiscal 2018, more than 521,000 people were apprehended according to CBP figures, up from 415,500 people taken into custody the previous year.

In the past, asylum-seekers taken into custody inside the U.S. have been processed and released pending further action. The Trump administration last month said that people seeking asylum in the U.S. must apply for the protection while still in Mexico rather than in the U.S.

