Central American migrants are taken into custody by U.S. Border Patrol agents in El Paso on Feb. 1. (John Moore/Getty Images)

U.S. border officials said Friday that they saw an abrupt drop-off in illegal crossings during the holiday season, but the number of Central Americans arriving in family groups has returned to record levels since then.

The lull in unauthorized crossings suggests some Central American migrants — and the smuggling organizations that deliver them to the border — may have taken a break or deferred the journey north until after the holidays.

During the first week of January, when the extended Christmas season was still being celebrated in Mexico and Central America, the number of migrant family members taken into custody by U.S. agents fell to as low as 200 on some days, according to preliminary data from U.S. Customs and Border Protection.

By the middle of January, the number of family members arriving jumped again, reaching as high as 1,400 per day, and smuggling organizations once more began delivering groups of 300 or more parents and children to remote border crossings in Arizona and New Mexico.

A CBP official told reporters Friday that about 75 percent of the family members detained in January arrived during the second half of the month. Family groups accounted for 59 percent of all border apprehensions, and although the total number of parents and children taken into custody — 24,116 — declined slightly from December, “that still represents one of the highest totals we have on record,” one official said.

Overall, CBP carried out 58,207 arrests and detentions in January, down 4 percent over the previous month, the latest figures show.

President Trump last spring ordered a “zero tolerance” prosecution push at the border that led to the separation of at least 2,500 families before he halted the measures amid a torrent of criticism. Since then, the number of parents arriving with children has accounted for an ever-growing share of border arrests, as the family groups arrive seeking to turn themselves in to U.S. agents and request asylum or some form of humanitarian protection.

The families are typically processed and released from federal custody after a few days, with a hearing in immigration court that may be months or years away. Critics deride this model as “catch and release” and say it is adding to a legal backlog that has pushed U.S. immigration courts to the brink of collapse.

The number of “family unit” members taken into custody is up 290 percent during the first four months of the government’s 2019 fiscal year, compared with the same period last year.

While migration trends have historically followed seasonal patterns, the holiday lull documented last month was especially pronounced.

Homeland Security officials say they are studying intelligence data to better understand why arrest numbers fell so sharply. The arrival of large groups of Central American families is once more placing strains on Border Patrol stations, officials said Friday.

On Thursday a group of 325 parents and children crossed the border illegally west of Lukeville, Ariz., according to CBP officials in the agency’s Tucson sector.

The crowd was first spotted by one of the agency’s remote cameras, and a CBP team arrived by helicopter to find a large group of people making a bonfire in freezing temperatures.

It was the latest in a series of mass border-crossings in isolated areas with few agents and minimal fencing. According to CBP, “the group illegally entered the country through an area where there is only a vehicle barrier designed to prevent crossings.”

The migrants told CBP officials that buses and trucks had dropped them off along a desert highway throughout the night, and the 325 crossed the border together at 8 a.m. to wait for Border Patrol agents to pick them up.

The CBP officials, who spoke on the condition of anonymity to share the data with reporters, said narcotics traffickers use these large groups as a diversion to move drugs into the United States while agents are busy processing families with children.

Trump ordered 3,750 additional military personnel to the border this week. Typically, the troops do not interact with migrants, but CBP officials said service members have provided some transportation and medical support to assist busy border agents.