WASHINGTON (AP) — Arrests for illegally crossing the U.S. border from Mexico fell more than 6% in April to the fourth lowest month of the Biden administration, authorities said Wednesday, bucking the usual spring increase.

U.S. officials have largely attributed the decline to more enforcement in Mexico, including in yards where migrants are known to board freight trains. Mexico won’t allow more than 4,000 illegal crossings a day to the U.S., Alicia Barcena, Mexico’s foreign relations secretary, told reporters Tuesday, down from more than 10,000 Border Patrol arrests on some days in December.

Migrants were arrested 128,884 times in April, down from 137,480 in March and barely half a record-high of 249,737 in December, U.S. Customs and Border Protection said. While still historically high, the sharp decline in arrests since late December is welcome news for President Joe Biden on a key issue that has nagged him in election-year polls.