EL PASO — U.S. Customs and Border Protection plans to build a massive migrant processing center here to reduce the time families are spending in small holding cells after they are apprehended on the Southwest border, U.S. Rep. Veronica Escobar (D-Tex.) said Thursday. Escobar said the facility would be able to house as many as 800 people at a time and could be open by spring.

The new El Paso processing center would be the Trump administration’s first major response to calls for an overhaul of outdated immigration detention practices on the border, and it would address concerns about how families who seek out Border Patrol agents to request asylum are held. Those families who cross the U.S.-Mexico border have been held for days in small cells designed decades ago to house single Mexican men for short periods after they were apprehended trying to sneak into the country.

The calls for new facilities to handle families, who last month made up the bulk of people apprehended at the border, intensified after two Guatemalan children died in December while in the custody of U.S. agents in the El Paso sector.

“It’s long overdue,” said Escobar, who brought several Democratic lawmakers to El Paso in recent weeks to view conditions in detention cells. She said CBP Commissioner Kevin McAleenan briefed her on the plans last week. “The government has known for almost five years that there are fewer and fewer Mexican single males coming across our border and more and more Central American families and unaccompanied minors fleeing Central America.”

CBP officials did not immediately respond to requests for comment Thursday about the plans for the processing center. The General Services Administration posted a bid request on Feb. 7 looking for a 250,000-square-foot facility on the western edge of El Paso. The lease term would be from five to 10 years, with an unspecified government agency taking occupancy by April 15.

Ruben Garcia, the director of Annunciation House, an El Paso nonprofit that assists migrants after they are released from federal custody, said families have been saying in recent days that they’ve been detained for a week or more in small cells that border crossers call “hieleras,” or iceboxes. He welcomed the plans for the new processing center.

“People tell me they were in holding cells eight, nine, 10 days,” Garcia said. “What that’s telling me is that CBP doesn’t have the personnel to process these families fast enough, so the number of days in these cells is creeping back up. After the two kids died, there was a concerted effort to try and not keep people in those holding cells for health reasons.”

The number of family units apprehended after crossing in the El Paso sector, which includes all of New Mexico, has risen sharply since October. From October through January, the first four months of fiscal year 2019, 25,710 members of family units have been apprehended by El Paso sector agents, compared with 12,312 in all of FY 2018 and 8,609 in FY 2017, according to CBP statistics.

The facility likely to be leased as a processing center was formerly used as a Hoover manufacturing center that closed in 2014, Escobar said. The 478,000-square-foot building sits on 33 acres just off Interstate 10 in western El Paso. The compromise border funding bill announced this week in Washington includes $190 million for a new El Paso processing center, she said, though she said it hasn’t been made clear whether that facility would be at the same location that CBP plans to lease.

The Trump administration has said that El Paso will be the next area targeted for its “Migrant Protection Protocols,” a policy that would require asylum seekers to remain in Mexico while their asylum cases are heard in U.S. courts. The policy has been implemented on a small scale in the San Ysidro-Tijuana area. It’s not clear why CBP would open a major processing center in El Paso if it intends to send asylum seekers back to Mexico.

On Thursday, Mexican officials confirmed that the Trump administration’s ambitious program to keep asylum seekers waiting across the border had expanded to include children.

Mexico’s migration institute said 10 children were sent back Thursday as part of a group that also included 53 adults. Three of the minors were under age 6, the institute said in a statement.

The Trump administration last month began sending asylum seekers back to Mexico to await their court proceedings, something that could take months or even years. The move is an effort to stanch a wave of migration by Central Americans who request protection from the U.S. court system. Homeland Security officials say many of the claims are specious, but migrant advocates note the Central Americans are fleeing countries plagued by violence.

Mexico’s immigration commissioner, Tonatiuh Guillén, had initially said the program, also known as “Remain in Mexico,” would only involve adults and one border crossing in California. The migration institute’s statement did not explain the turnabout, saying only that the asylum seekers were accepted back into Mexico “for humanitarian reasons.”

Immigration attorneys and human rights activists have protested that the new policy will leave asylum seekers waiting for long periods in border cities rife with violence. The American Civil Liberties Union filed on Thursday a lawsuit challenging the policy.

“The Trump administration is forcibly returning asylum seekers to danger in Mexico,” said Judy Rabinovitz, deputy director of the ACLU’s Immigrants’ Rights Project. “Once again, the administration is breaking the law in order to deter asylum seekers from seeking safety in the United States.”

The lawsuit was filed in federal court in San Francisco. The ACLU is joined in the lawsuit by the Southern Poverty Law Center and the Center for Gender & Refugee Studies.

Sheridan reported from Mexico City. Moore is a freelance journalist based in El Paso.