HAVANA, Tex. — There was about 200 pounds of marijuana in the bundles — hardly a monumental seizure by U.S. Border Patrol standards — but the agents who had chased the smugglers through the brush, nabbing one of them along with the drug haul, were almost giddy about the “regular traffic.”

“Everybody’s going to go, everybody and their mother,” Agent Salvador Pastran had predicted moments before, knowing that all of his colleagues would want to be part of the response when a call of “46” — code for drugs — came in over the radio. He took off at high speed in his truck. Now there were nearly a dozen Border Patrol agents and Army National Guardsmen gathered at the train tracks, the duct-tape-and-plastic-bound loot at their feet. The 17-year-old alleged drug runner was panting nearby, detained next to a truck.

“Regular traffic” — or sometimes “good traffic” — is how Border Patrol agents refer to the smugglers, drug couriers and others trying to sneak past them into the United States. It is, for many agents, their raison d’etre, the bread and butter of what the Border Patrol is supposed to be doing on a daily basis. It is what agents here say they’re finally able to focus on again, now that the overwhelming surge of migrant families crossing — considered “bad traffic” because few agents want to have to process parents with children — is over.

The number of migrants taken into custody along the U.S. southern border fell for the third consecutive month in August, dropping 22 percent from July to 64,006, acting U.S. Customs and Border Protection commissioner Mark Morgan said Monday. In a White House news conference, Morgan credited President Trump’s June 7 immigration enforcement agreement with Mexico as the key factor in the decline.

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 along its borders and highways to intercept Central American families as they headed north.

Now Border Patrol agents in the Rio Grande Valley, an epicenter of this year’s influx, say they’re seeing the effects. The share of migrants taken into custody who are families and children, including at official ports of entry, fell from 70 percent in July to 55 percent in August. And an expansion of detention space, coupled with the drop in numbers, has freed up more agents to get back “on the line” to pursue the criminals whom some officials lamented were being neglected as the agency processed detained families.

Like their boss, Morgan, agents who spoke to The Washington Post last week credited the sudden appearance of Mexican national guardsman on the opposite side of the Rio Grande, starting a few months ago, with the drop in border crossings.

“I don’t know what agreement happened, but the Mexican military is on the other side now,” Pastran said, referring to the national guard. “I mean, I’ve never seen it like that . . . like they’re active on the river.”

Morgan said Monday that the administration was “absolutely encouraged” by the declining arrests, “but we know these numbers could always spike upwards.”

“Make no mistake: Mexico has stepped up in unprecedented ways,” he said. But “I am concerned whether the government of Mexico, including our partners in the Northern Triangle countries, are going to be able to sustain the level of commitment they have.”

Mexico’s foreign minister, Marcelo Ebrard, is scheduled to meet with Vice President Pence and other senior Trump administration officials Tuesday.

Border Patrol statistics typically reflect a drop in apprehensions during the summer months, when crossings are particularly dangerous in the scorching heat. Despite the decline, arrests in August remained at their highest level in a decade — as compared with numbers from other Augusts — and the number of migrants taken into custody during the 2019 fiscal year, which ends Sept. 30, will probably reach nearly 1 million, a level not seen since the George W. Bush administration.

Homeland Security officials also say illegal crossings typically increase during the autumn months, and they will urge Mexico this week to do more to prevent a rebound.

Officials 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.

In the Rio Grande Valley, Border Patrol agents also are skeptical that the decrease in the number of crossings will last, in part because some areas where the Mexican authorities are working have historically been under cartel control. U.S. officials worry that the cartels can pay more than the Mexican national guard can.

“Whenever they’re in the area, you see a difference,” Border Patrol Agent Ryan Ansbro, who commands a unit patrolling the river by boat, said of the Mexican forces.

“I think all of the soldiers that are working here, they rotate them out very frequently because of all the corruption,” said Agent Deborah Villarreal, as she and Ansbro observed a group of Mexican troops on the riverbank.

Though Border Patrol agents say the Mexican authorities have been helpful, Morgan, the top border security official in the United States, said the country should not be so dependent on Mexico and other nations to compensate for a dysfunctional immigration system. He, like other Trump administration officials, blamed Congress for not taking action to cut down on immigration or to address what government officials see as loopholes allowing people to claim asylum — or at least secure their release into the United States while they await U.S. court hearings.

“We cannot rely solely on the government of Mexico or our Central American partners,” Morgan said, appearing to set the stage for short-lived gains from Trump’s Mexico deal. “. . . Unless the laws change, these numbers will rise again next year, just as we’ve seen in the past.” For the time being, “Congress continues to fail.”

White House officials have declined to say whether further tariff threats against Mexico are off the table. But Mexican officials say they have delivered on their promises to drive the numbers down.

“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 the CBP are the most widely used yardstick for measuring fluctuations in unauthorized migration. The figures include adults and children taken into custody by the Border Patrol between official crossing points, as well as those deemed “inadmissible” and detained after arriving at official border crossings, ports and airports.

Morgan told reporters that 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 Protection 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 is tracking reports of asylum seekers being extorted and kidnapped after being turned back because of the MPP program, Morgan said Mexico has provided “nothing to the United States corroborating or verifying those allegations.”

Morgan, whom Trump named acting commissioner in June but who 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 Monday that blocked an administration immigration policy. The judge reinstated a nationwide injunction that prevents the U.S. government from disqualifying asylum seekers who decline to seek protection in other nations while traveling to the U.S. border.

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

“Every single time that 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.

Miroff reported from Washington.