Between 2018 and 2019, for instance, the number of migrants in family units who crossed the border illegally jumped to 432,838 from 77,794, an increase of 456 percent. The number of apprehended migrants who were single adults climbed by 30 percent, to 258,375 from 198,492.

Last year, 621,311 family units were apprehended after crossing the southern border.

In recent years, Mexican families displaced by cartels that control swaths of territory have been crossing the border in ever-greater numbers to seek safety in the United States.

In the first eight months of the 2024 fiscal year, which started Oct. 1, the Border Patrol apprehended nearly 150,000 Mexican migrant families entering the United States illegally, compared with 87,014 in 2023 and 17,040 in 2020.

“Huge numbers of Mexican families have been coming, and they are easy to send back,” said Kathleen Bush-Joseph, a policy analyst at the Migration Policy Institute, a nonpartisan think tank, because they can be returned to their country on a bus.

The removal of families, and the exemption for unaccompanied minors, under the new restrictions is almost certain to lead to family separations as desperate parents decide to send their children alone, often with smugglers, she said.

In May last year, a 4-year-old child was dropped into the United States over the steel wall that separates San Diego from the Mexican city of Tijuana. The child survived. Two years earlier, agents rescued two young sisters, 3 and 5, who had been dropped on the U.S. side of the barrier in New Mexico.