(CNN) A list provided by the government suggests that fewer than half the migrant children younger than 5 years old who have been forcibly separated from their parents will be reunited with their families by Tuesday, the American Civil Liberties Union (ACLU) said.
A court order issued June 26 set a deadline of July 10 to reunite the roughly 100 young children with their parents.
On Friday, Justice Department attorney Sarah Fabian told a court that officials might need additional time to track down dozens of parents who are no longer in custody -- including 19 who were already deported. Fabian said officials should have no problem reuniting children and the 46 parents who remain in ICE custody.
Court will reconvene on Monday.
In a media release Sunday, the ACLU said that the government had "initially provided incomplete information" by a Saturday night deadline to list the children under 5 it had forcibly separated from parents and a revised list provided Sunday gave the names of 102 children.
"It appears likely that less than half will be reunited by Tuesday's court-ordered deadline," the group said.
A July 26 deadline remains in place for the reunification of all migrant children separated from their families by the Trump administration.
"It's extremely disappointing that the Trump administration looks like it will fail to reunite even half the children under 5 with their parents. These kids have already suffered so much because of this policy, and every extra day apart just adds to that pain," Lee Gelernt, deputy director of the ACLU's Immigrants' Rights Project, said in the statement.
But an administration official said efforts to reunite the children are "highly encouraging."
"Consistent with the court's optimism and encouragement, the administration has worked tirelessly since Friday's status conference toward the shared goal of promptly reunifying families while ensuring the safety of the children," the administration official said.
"The results of that work have been highly encouraging, and the Department of Justice is eager to present its progress to the court on Monday and to chart a path forward to safely reunifying other families expeditiously."
Friday's hearing was the latest in an ACLU lawsuit over the administration's family separations. The case includes a broader group of thousands of other children and parents, but the hearing focused largely on the pressing deadline for children under 5.
The Justice Department attorney provided the most detailed description officials have released so far about this particular group of immigrant children who were separated from their parents. Of the roughly 100 kids, Fabian said:
• 83 kids have been linked to 86 parents.
• 16 kids have not yet been linked with parents.
• 46 of those parents remain in the custody of Immigration and Customs Enforcement.
• 19 of those parents have been released from ICE custody.
• 19 of those parents have been removed from the United States.
• Two of the parents have criminal histories that will prevent officials from reuniting them with their children.
Officials have been marshaling "significant resources" as they work to comply with the court order, Fabian told the court Friday.
But she said their efficiency is hampered by requirements in the Trafficking Victims Protection Reauthorization Act, which sets out requirements officials have to follow when they release children into someone's custody.
It's one reason, she said, why officials are taking such painstaking steps to confirm parent-child relationships using methods that include DNA testing.
Gelernt took issue with the government's DNA testing, saying it should be used as "a last resort" and pointing to privacy concerns.
He argued the government does not have to follow such "lengthy, cumbersome procedures" to reunite separated families.