WASHINGTON — Arrests at the southwestern border dropped by 28 percent in June, according to the Department of Homeland Security, signaling the first time this year that the number of border crossings declined.
The department said that 104,344 arrests occurred in June, down from 144,278 in May — the highest monthly total in 13 years. It credited the drop to the security forces Mexico deployed to prevent migrants from reaching the United States border and the expansion of a program that forces migrants to wait in Mexico as their immigration cases are processed.
But immigration experts and former homeland security officials have cautioned that it is too early to determine the long-term effects of such enforcement measures. Border crossings typically increase in the spring and slip during the hot months of June and July.
“It is very hard to pin fluctuations in the numbers month by month to any particular factor, but we do know the numbers have gone down,” said Cecilia Muñoz, the director of the domestic policy council in the Obama administration. “We believe it has more to do with temperature in the desert than anything.”
President Trump, who has made fighting illegal immigration a centerpiece of his time in office, threatened in May to impose tariffs on all imported goods from Mexico. In response, Mexico promised to send troops to Mexico’s border with Guatemala, the point of entry for a flow of migrants who then make their way to the United States border.
The Department of Homeland Security said in a statement that the surge of migrants had overwhelmed facilities and resources along the border. The Border Patrol made more than 688,000 arrests through the end of June. In the 2018 fiscal year, the agency recorded more than 521,000.
The more than 100,000 arrests in June — the fourth straight month with that many crossings — is also more than double the 43,180 arrests in June 2018.
“We are past the breaking point and in a full-blown emergency,” the department said in its statement. “This situation should not be acceptable to any of us.”
The surge of migrants, which includes many asylum-seeking families from Central America, has slowed down processing, filled border facilities and riled Mr. Trump. While border officials could quickly deport people from Mexico — who made up the majority of those seeking to enter the United States when border crossings were at their peak in the early 2000s — immigration laws prevent the authorities from quickly deporting children from Central America.
Under federal court settlements, those children are not supposed to be held in facilities along the border for more than 20 days.
But Border Patrol agents, inspectors and lawyers say that many children have stayed in overcrowded, disease-filled facilities for weeks on end, even though policy requires that they be moved to shelters managed by the Department of Health and Human Services within 72 hours.
Homeland security officials have said that those facilities are also full, resulting in a backup in the system.
The Senate last month passed a $4.6 billion emergency aid package to improve those conditions.
Mr. Trump forecast the decline in migration in recent days, even celebrating Mexico’s efforts to crack down on immigrants.
“It’s had a tremendous effect, tremendous impact,” Mr. Trump said. “And you’ll see the numbers starting to come in very well.“