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.”