WASHINGTON — The number of migrants taken into custody along the U.S. southern border fell 22% — to 64,006 — in August, acting U.S. Customs and Border Protection commissioner Mark Morgan said Monday, crediting President Donald Trump’s June 7 immigration enforcement agreement with Mexico for a sharp drop in unauthorized crossings this summer.

Border arrests peaked at more than 144,000 in May, an influx that prompted Trump to threaten Mexico with crippling tariffs unless the government of President Andrés Manuel López Obrador took immediate steps to curb the flow.

Mexico responded by deploying thousands of national guard troops to ramp up arrests and intercept more Central American families heading north. Mexico’s foreign minister, Marcelo Ebrard, is scheduled to meet with Vice President Mike Pence and other senior Trump administration officials Tuesday to take stock of the enforcement effort.

Speaking to reporters at The White House, Morgan said the administration was “absolutely encouraged,” by the declining arrests, “but we know these numbers could always spike upwards.”

Morgan also said the percentage of migrants who arrive as part of family groups fell from 70% to 55% last month. The change is significant, he said, because parents who arrive with children are much more likely to be released into the U.S. interior after they are processed, rather than detained and deported.

Overall, border arrests are down 56 percent since the June 7 Mexico accord, Morgan said.

Despite the steep decline in crossings this summer, arrests in August remained at their highest level in a decade, and the number of migrants taken into custody during the 2019 fiscal year that ends Sept. 30 will probably reach nearly 1 million, a level not seen since the George W. Bush administration.

Homeland Security officials say illegal crossings typically increase during the autumn months as scorching temperatures ease, and they will urge Mexico this week to do more to avoid a rebound. Officials also have said that the percentage of northbound migrants arrested by Mexico is diminishing, in what they view as a sign of flagging efforts. “We need Mexico to do more,” Morgan said.

Morgan, the country’s top border security official, also said the United States should not be so dependent on Mexico and other nations to compensate for a dysfunctional U.S. immigration system.

“We cannot rely solely on the government of Mexico or our Central American partners,” Morgan said. “Unless the laws change, these numbers will rise again next year, just as we’ve seen in the past.”

Mexican officials say they have delivered on their promises to drive the numbers down, and they think Trump’s tariff threat should be off the table. White House officials have not said that, and in recent days having been trying to credit the President for securing the Mexico deal while also signaling that they are still not satisfied.

“Looking forward to meeting with Mexican gov officials tomorrow to talk about their recent efforts & discuss ways we can continue to secure the border,” Pence said Monday in a tweet. “There is still more work to do!”

The monthly arrest totals reported by CBP are the most widely used yardstick for measuring fluctuations in unauthorized migration. The figures include adults and children taken into custody by the U.S. Border Patrol between official crossings, or ports of entry, as well as those deemed “inadmissible” and detained at those entry points.

Morgan told reporters the United States has sent more than 42,000 asylum seekers back to Mexico to wait outside U.S. territory for their claims to be processed under an experimental program called the “Migration Protections Protocols” (MPP). He urged Mexican authorities to allow the Trump administration to send even more migrants out of the country, describing the program as a key deterrent.

Asked whether the administration was tracking reports of asylum seekers being extorted and kidnapped after being turned back by the MPP program, Morgan said Mexico has provided “nothing to the United States corroborating or verifying those allegations.”

Morgan, who was named acting commissioner by Trump in June but has not been nominated for the job permanently, also told reporters that he was “frustrated” by a U.S. District Court judge’s ruling in California on Tuesday reinstating a nationwide injunction blocking the administration’s attempt to disqualify asylum seekers who decline to seek protection in other nations while traveling to the U.S. border.

Morgan called Judge John Tigar’s injunction an example of “judicial activism.”

“Every single time this administration comes up with what we believe is a legal rule or policy that we believe that will address this crisis, we end up getting enjoined,” he said.