The U.S. government has stopped flying migrant families with children hundreds of miles across southern border states for the purpose of expelling them to Mexico amid mounting pressure and legal scrutiny from advocacy groups, Customs and Border Protection confirmed to CBS News. For several months, U.S. officials had been placing families who recently crossed the border in south Texas on planes and transporting them to El Paso and San Diego in order to expel them to Mexico from those locations. The policy was designed to circumvent the Mexican government’s refusal to accept Central American families with young children in the state of Tamaulipas, which borders Texas’ Rio Grande Valley, the busiest sector for unlawful crossings. CBS News has spoken to multiple Central American parents who crossed the Texas border with children as young as 2 years old and tried to ask for U.S. asylum. They said they were detained in Border Patrol facilities for days before being flown to San Diego, where U.S. officials expelled them to Mexico. The flights had been strongly criticized by advocates for migrants and asylum-seekers, who argued the policy undermined the public health justification behind the expulsions policy known as Title 42. The Biden administration has said the Trump-era order authorizing the expulsions is necessary to keep migrants from crowding government holding facilities and potentially spreading the coronavirus. Advocates said the fact that families were held in Border Patrol holding facilities with other migrants and then placed on buses and planes undercut the Title 42 rule’s stated objective. During a call with reporters on May 4, Homeland Security Secretary Alejandro Mayorkas told CBS News he was “taking a close look” at the cross-border flights. Despite the discontinuation of the flights, U.S. officials said they can still expel families with children to Mexico without allowing them to seek asylum. “The border is not open and CBP is still operating under Center for Disease Control and Prevention (CDC) guidelines for the COVID-19 pandemic. CBP is making every effort to remain within CDC guidelines and mitigate long periods of processing and holding to minimize potential exposure to our workforce, those in custody, and the community,” the agency said in a statement to CBS News. Migrants are processed by U.S. Border Patrol agents after illegally crossing the border over the Rio Grande River in Roma, Texas, U.S., on Tuesday, April 28, 2021. Eric Thayer/Bloomberg via Getty Images Some families encountered in the Rio Grande Valley are still being transported to Laredo, Texas, and expelled to Mexico from there, CBP said. Certain families who crossed into the U.S. near Yuma, Arizona are being transported to San Diego, from which some are expelled to Mexico, the agency said. “Several Border Patrol Sectors have seen an increase in encounters. In order to process individuals as safely and expeditiously as possible, other Sectors along the Southwest border are assisting by processing these subjects at their facilities,” CBP said in its statement. Sister Norma Pimentel, who runs the biggest shelter for migrant families in south Texas, said many families encountered in that area have been transported by U.S. officials to Laredo in recent days. She said some are expelled to Mexico from there, while others are released to local shelters, noting the Mexican government has a daily cap on expulsions. During President Biden’s first three full months in office, U.S. authorities along the border with Mexico have summarily expelled more than 48,000 migrant parents and children traveling as families under the Title 42 authority, according to government data. At the same time, however, the Biden administration has been turning back a smaller percentage of the families crossing the southern border than the Trump administration did. In April, for example, 65% of parents and children traveling as families were processed under regular immigration laws and allowed to request U.S. sanctuary. Most of those families are released to local communities and shelters, where they are tested for the coronavirus before boarding buses and planes to their respective destinations in the U.S. In addition to Mexico’s reluctance to accept families with children six and younger in Tamaulipas, Biden administration officials have said some families are exempted from the Title 42 process because of medical conditions and other “acute vulnerabilities.” Hundreds of migrants from Honduras, Guatemala and El Salvador wait to register names upon their arrival in the U.S. after crossing the Rio Grande river from Mexico aboard, in Roma, Texas, United States on April 9, 2021. Tayfun Coskun/Anadolu Agency via Getty Images CBP did not elaborate on why it discontinued the cross-border flights, only saying the decision was made “based on operational needs.” However, the policy change comes as the use of the Title 42 authority on families with children continues to be the subject of ongoing negotiations between the Biden administration and the American Civil Liberties Union, which filed a lawsuit in January in a bid to halt the expulsions. In February, the ACLU agreed to pause its lawsuit on behalf of migrant families to negotiate with the government. Since then, the two parties have agreed to continue the suspension of the court case multiple times. However, no resolution has yet been reached and the current pause expires Thursday at midnight. Lee Gelernt, the lead ACLU lawyer in the case, said his group remains concerned about families being bused to different sectors of the border just to be expelled back to Mexico. “We are making progress in negotiations getting the most vulnerable families to safety through a variety of means, but whether we resume litigation will ultimately depend on the administration giving us an acceptable end date for Title 42 altogether,” Gelernt told CBS News.