The number of people taken into custody along the U.S. southern border fell 28 percent 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.