All migrant children under age 5 who are eligible will be reunified with their parents by early on Thursday morning, a Trumpadministration official said on Wednesday.
The administration faces a July 26 deadline to reunite the children older than 5.
It was the largest single effort to date to undo one effect of President Donald Trump's zero-tolerance policy - separating families who try to slip across the Mexican border into the country.
According the the U.S. Department of Health and Human Services, only four kids were unified before Tuesday's deadline.
One of the biggest operators of migrant-youth shelters in the United States, Southwest Key Programs, said its staff had dispatched several children from its shelters to return to their parents on Tuesday.
Those 57 children represent more than half of 103 "tender age" juveniles who had been identified as separated from their parents in a court case the American Civil Liberties Union filed against the government.
Now the family detention system is at capacity - as are a network of about 100 shelters for migrant minors who traveled into the US without parents or a guardian and who now wait anywhere from a few weeks to years to have their cases resolved in the immigration courts. The government argued that the progress showed they were in compliance with the court order, even if the deadline was missed, but plaintiffs argued otherwise.
The families that have been reunited were also released from detention while they undergo immigration proceedings, which could ultimately lead to them being deported, so long as they do not commit crimes and do attend court hearings and meetings with immigration agents.
Eleven children are in state or federal custody for other reasons, while 12 adults who were separated from children at the border have already been deported. The ACLU claims that the government initially provided incomplete information about the children.
"Let me be clear: HHS could have transferred every child out of our care to a parent who is now in DHS custody today if we did not take into account child safety or whether the adult is actually the parent", Chris Meekins of Health and Human Services Department said in the call. And one adult each had a "falsified birth certificate", was "alleged to have abused the child" and was "being treated for a communicable disease".
Government officials touted the screening process for parents, including background checks and DNA tests. "Defendants are unable to conclusively determine whether the parent is a class member, and records show the parent and child might be US citizens".
"If in fact 57 children have been reunited because of the lawsuit, we could not be more happy for those families", Lee Gelernt, deputy director of the ACLU's Immigrants' Rights Project said in a statement.
The US government has said some children were not eligible for reunification because their parent had either been deported, had a criminal record or was otherwise unfit. A lawyer for the US said that of the 102 children under five, the USA has determined 54 will be rejoined with their families. They may be reunited with children once they are located and contacted.
Still, Sabraw said that despite their differences, both sides were "all rowing in the same direction" in their efforts to end family separations at the border.
The government has said it is taking care to ensure children are safe by checking parents' criminal history, testing DNA to prove family relationships and reviewing adults for their suitability as a caregiver.
Late last month, Sabraw, an appointee of Republican President George W. Bush, set a 14-day deadline to reunite children under 5 with their parents and a 30-day deadline for older children. Only now, they'll do it as a unit rather than separately.
Gelernt told the NewsHour on Tuesday that the ACLU hopes "that now the government has figured out some basic process for doing these reunifications".