The Department of Homeland Security projects that arrests of migrants along the southern border will fall 25 percent this month, a drop that authorities attribute to a Mexican crackdown on Central American migrants and the expansion of an experimental Trump administration program that requires asylum seekers to wait outside U.S. territory for their immigration court hearings.

U.S. authorities detained more than 144,000 migrants in May, the highest monthly total since 2006. After President Trump threatened to impose tariffs on imports from Mexico, the government of President Andrés Manuel López Obrador agreed to deploy thousands of troops to intercept migrants, prevent illegal crossings from Mexico into the United States, and host more asylum seekers as they navigate the U.S. courts.

“It’s become clear that over the past three weeks, since the administration reached a new agreement with Mexico, that we’ve seen a substantial increase in the number of interdictions on the Mexican southern border and a sincere effort to address the transportation networks coming through Mexico,” acting homeland security secretary Kevin McAleenan told reporters Friday in Washington.

McAleenan praised lawmakers for providing $4.6 billion in supplemental funds for border initiatives — a rare bipartisan border agreement reached Thursday — and he said the money immediately would be applied to improve conditions in overcrowded processing centers and detention facilities along the southern border.

Even with the drop, McAleenan said a 25 percent decline was “not acceptable,” as U.S. authorities were hoping for a far greater impact from the new efforts to keep people away from the border. DHS authorities said they would not get a fuller picture of the results of Mexico’s efforts for several more weeks.

“In terms of when we’re going to know if these efforts in Mexico are making an impact, I think these three weeks have demonstrated that they are already,” McAleenan said. “That 25 percent decrease in June is more than we’ve seen in past years.”

McAleenan also said that historic seasonal patterns were less relevant to current migration trends. He and other administration officials say some of the influx is related to deficiencies in U.S. immigration laws and “loopholes” that generally allow adults who arrive with children to avoid detention and deportation.

“It’s more about the pull factors, in my experience, than traditional weather or agricultural or seasonal workers. This is a different situation,” he said, calling for further action from lawmakers to tighten U.S. regulations. “So I think we’re going to know basically by mid-July and certainly by the end of July whether these efforts are sustained and having a significant impact.”

The U.S. program of keeping people in Mexico as they await their asylum hearings, known as “Remain in Mexico” or the Migrant Protection Protocols, has come under federal court challenge. While U.S. authorities have been expanding it, sending thousands of people back into Mexico after their arrival across the U.S. border, the Court of Appeals for the 9th Circuit in California is weighing the program’s legality.

The U.S. government has returned more than 13,000 migrants to Mexico this year via MPP, according to a DHS official.

On Wednesday, the union that represents U.S. asylum officers asked the federal court to end the program, arguing that the policy is threatening migrants’ lives by forcing them to stay in dangerous Mexican cities where they can face persecution and claiming that it is “fundamentally contrary to the moral fabric of our Nation.”

The majority of people apprehended crossing the border illegally are families or unaccompanied minors from Central America, many of whom say they are fleeing violence and extreme poverty. Children, many younger than 12, accounted for about 40 percent of apprehensions in May.

Four children died during the past week while trying to reach the United States, McAleenan said, including a toddler named Valeria who drowned in the Rio Grande with her father, Óscar Alberto Martínez Ramírez, of El Salvador. Their deaths were captured in an image published by La Jornada and later by the Associated Press. McAleenan said the deaths “should not be acceptable for any of us.”

The administration also has faced scrutiny over the conditions that children are experiencing at U.S. Border Patrol facilities that were never designed to hold them. Lawyers have described what they found to be “appalling” conditions in the Clint Border Patrol facility in Texas, including small children without diapers and older children caring for younger ones. McAleenan called the allegations “unsubstantiated” and said the facility is “clean and well managed.”

During a tour of the Clint facility this week, Border Patrol officials said children receive adequate food and care but emphasized that their facilities are not meant for long-term holding of anyone, let alone children.

McAleenan said he supports border and interior enforcement of U.S. law, but he did not indicate whether DHS plans to go forward with an operation to target migrant families that have deportation orders, something Trump publicized and then postponed for two weeks on June 22.