TUCSON — Concerned about the rising number of migrant families crossing from Mexico into the Arizona desert, Department of Homeland Security officials are preparing to bus border crossers more than 300 miles east into Texas so they can deposit them in Mexico instead of releasing them in the United States, according to two Trump administration officials.

Homeland Security officials said Friday that they will expand the Migrant Protection Protocols (MPP) program to the Tucson region, one of the last major areas on the border that has not been diverting asylum seekers to Mexico to await their U.S. immigration court hearings.

Officials deployed the program after a modest uptick in family crossings in the Tucson sector at a time when the number of migrants crossing the border is declining almost everywhere else. Officials view MPP as their chief line of defense on the border and worried that smugglers had identified Tucson as a weak spot because officials have been unable to stand up shelters and court hearings on the border in Arizona.

DHS spokeswoman Heather Swift said DHS is “strengthening” MPP in Tucson and in the Del Rio, Tex., sector, which started sending migrants to Mexico last month. She did not respond to requests for additional details about the plans.

“The department is continually assessing MPP and making operational changes in response to emerging trends and threats, and we always consider additional return points and options,” Swift said in an email. She said smugglers “are sophisticated organizations that are constantly trying to get around the effective tools we have in place across the border.”

Officials estimate DHS will send at least one busload each day from U.S. Customs and Border Protection’s Tucson sector to the Texas border city of El Paso, according to the officials, who spoke on the condition of anonymity to discuss the internal plans. Migrants will have interviews to determine if they would be at risk in Mexico, and if not, they will be sent to Ciudad Juárez to await their U.S. immigration court hearings. The first bus was scheduled to run to El Paso on Friday.

CBP’s Tucson sector, which covers a swath of forbidding desert and cactus forests that encompass most of Arizona’s southern border, was not included in the initial rollout of the policy.

Federal officials said they had not expanded MPP to Tucson and other sectors because they had fewer attempted crossings there than El Paso and other locations along the border. Officials also must negotiate the returns with Mexico, which agreed to support the program in June after Trump threatened to impose tariffs.

CBP’s Tucson sector cannot send migrants directly across the border because the U.S. side does not have court hearing rooms and the Mexican side does not have sufficient shelter space for migrants, officials said.

“The infrastructure to implement MPP doesn’t exist in Arizona and both governments are working on a solution, but that hasn’t happened yet,” said a senior Border Patrol official who spoke on the condition of anonymity to describe internal details of the program. “There simply isn’t the capacity in place to set up MPP and neither government is prepared yet to do this in Arizona.”

Similar issues have cropped up in other border cities. The Del Rio sector, east of Arizona, can send up to 50 migrants a day into Mexico through two U.S. cities, Eagle Pass and Laredo, a spokesman said.

DHS officials began the MPP program in January but substantially expanded it after a federal appeals court ruling allowed it to move forward in May. The U.S. Court of Appeals for the 9th Circuit is expected to rule soon on the broader legality of the program.

Federal officials have credited MPP and increased enforcement in Mexico with a dramatic plunge in border apprehensions in recent months.

Nearly 1 million migrants were taken into custody along the border during the most recent fiscal year, including a record number of migrant families and unaccompanied minors, mostly from Central America. The White House said adults traveled with children because they were able to quickly bypass immigration custody and were released to await court hearings.

But since the Trump administration and Mexico ratcheted up enforcement, the number of people taken into custody has dropped from 144,000 migrants in May to 45,000 last month. In September, officials said border crossings had fallen so much that it allowed them to “effectively end” the practice of releasing migrants into the United States to await court hearings.

The practice had frustrated President Trump because migrants who were allowed to stay in the United States rarely are deported, even if they lose their bids to stay in the country. Officials say many migrants do not show up for their court hearings; some skip out on the hearings, and others say they were never informed about them.

As family apprehensions plunged elsewhere, they rose more than 33 percent from May to October in the Tucson sector, from almost 1,800 to nearly 2,400. Border Patrol officials continued releasing families into the Tucson sector, and smugglers caught on, U.S. officials said. The Wall Street Journal first reported the releases earlier this month.

Family arrivals in Tucson remain far lower than the tens of thousands who poured into other parts of the border earlier this year, particularly in El Paso.

But Tucson’s crossings in October surpassed those in El Paso, which counted 2,100.

More than 200 people — ­including one large group of 129 people — streamed into remote Sasabe, Ariz., in the Tucson sector in a span of five hours on Saturday night. Babies swaddled in scarves cuddled against their mothers, according to images CBP released.

The migrants were from Mexico, Guatemala, El Salvador, Honduras and Nicaragua. The oldest was 56. The youngest was 6 months old.

Advocates for immigrants say migrants’ willingness to trek to the Arizona desert is a sign of their desperation to escape their homelands and of their fear of waiting in Mexico’s high-crime border cities for an asylum hearing.

Speaking at a gathering of faith leaders Thursday in Tucson, the Rev. Rodger Babnew said some migrants have come from Matamoros and Ciudad Juárez after learning that they could enter the United States via the Tucson sector. He said criminal cartels have begun shaking down migrants for money along the route.

“They come because they know they will be returned to Mexico,” said Babnew, an Episcopal minister with a nonprofit organization called Cruzando Fronteras, or Crossing Borders, which shelters and provides medical care to migrants on both sides of the border. “People are trying to get to their families before the holidays.”

Miroff and Sacchetti reported from Washington.