Washington — For the first time in more than a year, the U.S. is apprehending fewer migrant children and families than adults along the southern border, a major demographic shift that Trump administration officials attribute to "consequences" they are applying to deter U.S.-bound migrants.

U.S. border officials in October apprehended more than 35,000 migrants — including nearly 10,000 families and 3,000 unaccompanied migrant children — along the U.S.-Mexico border, marking the fifth consecutive monthly decline in arrests there, Customs and Border Protection (CBP) officials said Thursday.

Single adult migrants comprised the bulk of those apprehended at nearly 23,000 arrests. In fiscal year 2019, CBP officials said about 65% of migrants encountered at the southern border were families and unaccompanied children. In October, those populations constituted about 35% of all apprehensions.

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September and October were the first months when the number of apprehensions of adults surpassed those of unaccompanied children and families since August 2018.

Young asylum seekers play close to the International Bridge in Matamoros, Tamaulipas, on June 26, 2019. Veronica G. Cardenas / Getty Images

In fiscal year 2019, the U.S. encountered more than 76,000 unaccompanied migrant children at the southern border, including more than 12,000 in May. Most of them were transferred to shelters overseen by the Department of Health and Human Services, which handles the long-term care of unaccompanied migrant children until a sponsor is found or the child turns 18.

CBP officials said their officers are conducting significantly fewer rescues and encountering fewer large groups of migrants in recent weeks. Some of the support personnel that was brought in during the height of the surge of Central American migrant families in the spring have returned to their posts throughout the U.S., the officials added.

The catalyst for the continued decline, the officials said, is that the U.S. is now "applying consequences" to border-crossers and not allowing them to continue their immigration proceedings in the U.S. or outside detention.

"At height of crisis, CBP apprehensions exceeded 5,000 in a single day. Now, we're averaging just over 1,300. And we all but ended catch and release," Acting CBP Commissioner Mark Morgan said during a White House briefing Thursday. "Migrants can no longer expect to be allowed into the interior United States based on fraudulent asylum claims."

Acting Customs and Border Protection director Mark Morgan speaks with reporters in the briefing room at the White House on November 14, 2019. Alex Brandon / AP

Since apprehensions of migrant families reached at 13-year monthly high in May with over 130,000 apprehensions, border arrests have dropped for five consecutive months. In this time span, the Trump administration aggressively expanded its controversial Migrant Protection Protocols (MPP) program, requiring more than 55,000 asylum seekers to wait in dangerous Mexican border cities, where many live in squalid conditions, for the duration of their court proceedings.

The administration has also continued to limit the number of would-be asylum seekers who can present themselves at official ports of entry to claim fear of persecution through a practice known as "metering."

Last month, officials started implementing an pilot program along the El Paso sector of southern border designed to fast-track the processing and deportation of asylum seekers subject to a sweeping regulation allowed by the Supreme Court which renders most non-Mexican migrants ineligible for asylum.

On the international stage, the Mexican government has ramped up it immigration enforcement and deployed National Guard units to its borders with Guatemala and the U.S. to intercept migrants. The Trump administration has also brokered bilateral agreements with Guatemala, Honduras and El Salvador that would allow the U.S. to reroute asylum seekers to these countries, but none of the accords have taken effect yet.

Despite their success in deterring Central American families and unaccompanied migrant children, the CBP officials on Thursday noted that for the first time in 18 months, most of the migrants apprehended by U.S. border officials were of Mexican origin. The new trend is problematic for officials as Mexican migrants can't be placed in the MPP program or be subject to the asylum restriction allowed by the Supreme Court.

Immigration lawyers attempt to inform hundreds of asylum seekers of their legal rights on August 24, 2019. Michael Nigro/Sipa USA via AP Images

The officials also stressed that the experimental policies they've implemented in recent months to deter migrants are "short-term fixes" — and that court rulings could hamper their efforts if there is no congressional action to close what they see as "legal loopholes." Both the MPP program and the third-country asylum ban are being challenged in court.

"If some of the programs are challenged and we lose the ability to apply consequences, we're going to be right back in the same situation that we were in May," one official said.