Video footage the outlet obtained shows the hundreds of migrants walking alongside the US-Mexico border barrier near Yuma, and photos showed a small hole that Border Patrol agents said the migrants had entered through.
"In my 30 years with the Border Patrol, I have not been part of arresting a group of 376 people," Anthony Porvaznik, the Border Sector Chief in Yuma, Arizona, told ABC. "That's really unheard of."
Porvaznik added that the group included 179 children, more than 30 of whom were traveling without their parents. He said the most urgent solution his sector needed was more funding to provide for the families, though better border barriers was also important.
"That's our No. 1 challenge that we have here in the Yuma sector, is the humanitarian problem," he said. "As I mentioned, 87% of the apprehensions here are family units and unaccompanied alien children."
Customs and Border Protection did not immediately respond to INSIDER's request for comment.
Migrants tunneling under the barriers isn't uncommon, particularly in the Yuma sector, where Border Patrol agents spotted a group of 83 migrants digging under the barrier near the San Luis port of entry in November.
"Unlike most areas along the border, Yuma has some very sandy areas that is easy to dig in," Brandon Judd, the president of the National Border Patrol Council, told CNN .
CBP attributed the November tunnel to the fact that the border barriers in that sector were built using an outdated model.
"This old portion of wall lacks the improved concrete footer that new wall prototypes have which prevents easy digging underneath," the agency said in a statement .
Judd added that the group of 376 migrants arrested this week was "the largest I've ever heard of."
The news comes amid a month-long government shutdown over President Donald Trump's demands for a border wall. Trump has said the most effective way to prevent migrants from entering illegally is through border barriers, though his Democratic opponents have cited the tunnels as evidence the wall won't work.