A mother and 5-year-old daughter from Honduras are detained by U.S. Customs and Border Patrol agents Wednesday, July 18, 2018, in San Luis, Ariz. The pair were apprehended by a U.S. Border Patrol agent who spotted them crossing a canal along the border with Mexico. Credit: Matt York | AP

The number of migrant families taken into custody along the U.S. border with Mexico remained nearly unchanged from June to July, according to government data released Wednesday, an indication the Trump administration’s controversial move to separate thousands of parents and children did little to deter others from attempting the journey.

U.S. border agents arrested 9,258 “family units,” along America’s southwest border last month, down slightly from 9,434 in June and 9,485 in May.

The administration cited a springtime surge of parents crossing illegally with children as justification for its “zero tolerance” initiative, which led to the separation of approximately 2,500 families between May 5 and June 20, when public outcry forced President Donald Trump to end the practice.

Since then, some of the policy’s defenders have argued the separations would have had a stronger deterrent effect if allowed more time. They insist its true impact would not be apparent until word of the crackdown had spread to rural Central America, prompting parents to reconsider travel plans.

But the July arrest totals released Wednesday suggest the separations made little difference. While families continued to arrive at roughly the same rate, the number of unaccompanied minors taken into custody dropped from 5,093 in June to 3,938 in July, even though that group wasn’t a target of the “zero tolerance” crackdown.

A Department of Homeland Security senior official said the agency hasn’t concluded why there were fewer apprehensions of unaccompanied minors. But the official noted that, in July, family groups accounted for a larger share of unauthorized border-crossers – 29.6 percent.

“The fact that we’re unable to detain family units is attracting more people to cross as family groups,” said the official, referring to court-imposed limits on the government’s ability to keep migrant children in detention for longer than 20 days.

In total, U.S. agents took 39,953 migrants into custody along the border in July, down from 42,838 the previous month. Those figures were significantly lower than the 50,000 or more arrests made each month in March, April and May, an increase that left Trump fuming at Homeland Security Secretary Kirstjen Nielsen and demanding tougher measures.

Illegal migration along the Mexico border typically increases in spring before falling again during the summer when temperatures peak, raising the risk of heat stroke and death from exposure.

Families who cross illegally, most of whom are from Central America, typically float or wade across the Rio Grande, then turn themselves in to Border Patrol agents to claim asylum, citing fears from gang violence and lethal threats back home.

The Trump administration has moved to tighten asylum rules and says many of those seeking refuge are economic migrants trying to take advantage of the backlog of cases in U.S. immigration courts, gain entry to the United States and work in the country as long as possible.

One place where the government noted a decline in the number of families arriving was at U.S. border crossings, or ports of entry, where migrants can petition for asylum without crossing illegally.

More than 5,000 families per month arrived to the ports of entry in March and April, but in June and July that number fell to around 3,000.

The senior DHS official blamed advocacy groups for telling migrants that they would have their children taken away if they attempted to seek asylum at ports of entry. “That is the appropriate way to arrive to the country, but unfortunately many of the family units are in the grips of human smugglers,” the official said.

Another reason may be that U.S. officials have turned away more asylum seekers at official border crossings. In recent months, some families attempting to approach the ports of entry have been intercepted by U.S. Customs and Border Protection officers and told to come back later when there’s more capacity to handle their claims.

CBP said that all of its border crossings remain open to those who lawfully pursue asylum claims, but the ports do not always have the means to process them quickly, especially during the summer travel season, and at a time when traffickers are attempting to smuggle large amounts of narcotics into the United States.