The number of people taken into custody along the U.S. southern border fell 28% in June, a drop that U.S. authorities say reflects the early impact of Mexico's crackdown on Central American migration.

WASHINGTON - The number of people taken into custody along the U.S. southern border fell 28% in June, a drop that U.S. authorities say reflects the early impact of Mexico's crackdown on Central American migration.

Border crossings typically rise in the spring and slump during the scorching summer months, but the drop registered from May to June was significantly larger than in previous years, according to Homeland Security statistics released Tuesday. U.S. authorities detained 104,344 along the border last month, down from 144,278 in May.

June was the fourth month in a row that border arrests exceeded 100,000, and the total was more than twice the 43,180 taken into custody in June 2018 and a nearly fivefold increase over June 2017, when authorities detained 21,673.

President Donald Trump has treated the monthly U.S. Customs and Border Protection arrest totals like a stock index for the success of his immigration policies, periodically erupting in fury at Homeland Security officials as the numbers soared to a 13-year high.

In late May, with holding cells along the border overflowing and Central American migrants streaming across in groups of as large as 1,000, Trump forced emergency negotiations with Mexico by threatening to impose potentially crippling tariffs - a political gambit aimed at shifting responsibility for the border crisis to a foreign government.

The move spurred immediate action: Leftist president Andres Manuel Lopez Obrador's administration convinced Trump to delay the economic penalty by promising that Mexico would dramatically increase enforcement efforts and work with the United States to overhaul regional asylum policies.

Mexico has since deployed thousands of national guard troops to patrol its borders and interdict migrants traveling along railways and roads, at times grabbing families just steps from U.S. soil along the banks of the Rio Grande. Mexico said it has increased deportations 33% since the deal.

"The reduction in apprehensions accounts for decreases across all demographics, including unaccompanied minors, family units and single adults, as well as decreases in migrants from all Northern Triangle countries, particularly those coming from Guatemala," the Department of Homeland Security said in a statement.

"Decreasing apprehension numbers will provide greater opportunities for the DHS to address capacity challenges for those in custody and speed the movement of unaccompanied children into Health and Human Services (HHS) care," the department said.

Trump purged much of the DHS leadership this spring, including then-Secretary Kirstjen Nielsen, frustrated that she had not been able to stem the influx of migrants at the border. He installed McAleenan, the country's top border security official, and turned to him early last month to outline an enforcement strategy with Mexico that could bring the crisis under control.

The drop in border crossings has allowed for a bit of a reprieve for U.S. border stations, which officials say have been brought back from "the breaking point," allowing U.S. agents to improve care and processing times for children and families following a wave of anger over images of migrants packed into squalid cells and reports of mistreatment. The number of children in CBP custody has fallen from more than 2,500 in early June to fewer than 350 in recent days, DHS officials said this week.

U.S. Customs and Border Protection officials say the number of migrants its custody has dropped more than 40%, but the agency still has about 10,000 in holding cells and stations designed for half that many people.

On Wednesday, Democrats from the Subcommittee on Civil Rights and Civil Liberties plan to host a hearing titled "Kids in Cages: Inhumane Treatment at the Border" to question the impact of Trump's immigration policies.

While it likely will take months to see whether Mexico's enforcement efforts will have a sustained effect on migration, U.S. officials said the June numbers appear to be a first step toward controlling what is widely considered a humanitarian crisis that has been overwhelming the U.S. immigration system.

Homeland Security officials say they expect the number of arrests to continue falling through July. Trump has given Mexico until July 22 to demonstrate a commitment to arrest and deport more migrants, and he has issued statements in recent days suggesting his tariff threat has diminished.

"The southern border is being policed very well by Mexico," Trump told reporters Friday. "You'll see the numbers starting to come in very well."

But given the migration patterns of the past two years, the one-month dip also could just as easily reverse. Word has gotten back to Central America that Mexico is difficult to traverse right now, said Andrew Selee, director of the Migration Policy Institute in Washington, who visited El Salvador last week.

"You can pause the migration flow by beefing up enforcement resources," Selee said. "But the question is whether this will last."

Mexico's goal was to "take the tension out of the relationship" and placate Trump, he said. "Mexico has thrown every resource they could at immigration enforcement, and it's been chaotic and not entirely coherent, but in a way it's worked to accomplish what they want it to."

Higher summer temperatures often, though not always, have brought a seasonal dip in border arrests, and last year detentions fell 17% between May and June. McAleenan and other Homeland Security officials argue that seasonal patterns are less relevant now, amid a migration wave they say has been driven by parents bringing children to take advantage of "loopholes" in U.S. immigration laws and a dysfunctional asylum system.

Selee said the declining numbers are also significant because they "break the momentum" of a migration wave that has been building for months. "It certainly allows for some breathing room so that rational people on both sides can have conversations about long term solutions," he said.

Nearly 700,000 migrants have been detained along the U.S. border during the first nine months of the 2019 fiscal year, a total not seen since 2007, when the majority of those arrested were Mexican adults who could be quickly processed and deported.

Those arriving today are far more likely to seek out U.S. agents and assert fear of persecution, the first step toward initiating the process of seeking asylum. Because of court restrictions on the amount of time that minors can be held in immigration jails, parents who arrive with a child are typically issued a court appointment and released into the interior of the United States.

The Trump administration has begun a series of experimental policies to avoid such releases, insisting too many of the migrants are seeking to game the system and lack legitimate asylum claims.

The pact with Mexico has also allowed the United States to send more asylum seekers back across the border as part of the "Remain in Mexico" policy that requires applicants to wait on foreign soil.

At least 15,000 asylum seekers have been returned to Mexican cities to await their asylum hearings this year, according to Mexican officials. Some have accepted government-sponsored bus rides back to Central America, but shelters and tent camps in Mexican border cities remain crowded, and there have been widespread reports of families stranded in dire conditions. A growing number of African asylum seekers are among them.

The United States on Tuesday began sending asylum seekers back to the Mexican state of Tamaulipas, one of the country's most dangerous, to wait for their hearings.

The first families - 12 Cubans and Venezuelans - were sent back with notices for asylum hearings scheduled in September. They were not transported to shelters and several wandered the streets of Nuevo Laredo looking for help.

Local officials warned that they lacked the resources to care for the migrants and said they should sent further south, away from the border.

"This border is a very difficult place to deal with the migrants," said Salvador Rosas, a congressman from Tamaulipas. "We think it would be better to create a shelter 60 or 100 miles south."