Nearly 400 people were arrested in Arizona, after digging short, shallow holes under a barrier to cross the border, authorities say.
The group dug under a steel barrier in seven spots about 16km east of a border crossing in San Luis and made no effort to elude immigration agents. They included 176 children.
The unusually large group was almost entirely from Guatemala. They were taken to Yuma after entering the country Monday.
The area became a major corridor for illegal crossings in the mid-2000s, prompting the federal government to weld steel plates to a barrier made of steel bollards that had been designed to stop people in vehicles, not on foot, Border Patrol spokesman Jose Garibay III said. In those spots, there is no concrete footing to prevent digging.
The group used multiple holes in an apparent effort to get everyone across the border quickly, Mr Garibay said.
Customs and Border Protection released photos and video of a long line of migrants standing patiently on a desert road's dirt shoulder after they were stopped.
On Wednesday, the Border Patrol arrested a group of 247 people, who turned themselves in to agents in a highly remote part of New Mexico, where authorities have found 25 groups of more than 100 people since October. A group of 115 was found in the same area Thursday.
Large numbers of Guatemalan families and unaccompanied children are surrendering to immigration agents in Antelope Wells, New Mexico, where 7-year-old Jakelin Caal and her father were found 6 December with 161 others.
Jakelin started vomiting on the bus ride to the nearest Border Patrol station 150km away and had stopped breathing by the time she arrived. She died at a hospital in El Paso, Texas.
The southwest Arizona desert is less remote but arrests have also sharply increased after years of relative quiet. The Border Patrol's Yuma sector made 7857 arrests in October and November, more than double the same period a year earlier.
Despite a surge in asylum-seeking families from Central America in recent months, border arrests remain low by historical standards.
The Border Patrol made 396,579 arrests on the Mexican border in the 2018 fiscal year, up 30 percent from a 46-year low during the same period a year earlier but still well below a high of more than 1.6 million in 2000.