(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.

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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."

Where are the parents?

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.

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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.

Lengthy process of reunification

Photos: Immigration protests around the country Photos: Immigration protests around the country Salt Lake City: Activists hold signs Saturday to protest the Trump administration's approach to illegal border crossings and separation of children from immigrant parents. Hide Caption 1 of 16 Photos: Immigration protests around the country Chicago: Protesters fill Daley Plaza to listen to speakers and show opposition to the White House's immigration policies. Hide Caption 2 of 16 Photos: Immigration protests around the country Los Angeles: A woman attends a protest outside the Metropolitan Detention Center, after marching to decry Trump administration immigration and refugee policies. Hide Caption 3 of 16 Photos: Immigration protests around the country Indianapolis: An activist holds a poignant protest sign at the Indiana Statehouse. Hide Caption 4 of 16 Photos: Immigration protests around the country Los Angeles: A woman with "we care" written on her hands lifts them up at a protest outside the Metropolitan Detention Center. Hide Caption 5 of 16 Photos: Immigration protests around the country New York: People march across the Brooklyn Bridge in support of families separated at the border. Hide Caption 6 of 16 Photos: Immigration protests around the country Chicago: A young girl holds a sign as she takes part in the protest. Hide Caption 7 of 16 Photos: Immigration protests around the country Washington: A protester reacts to speakers in a crowded Lafayette Square across from the White House. Hide Caption 8 of 16 Photos: Immigration protests around the country Boston: US Sen. Elizabeth Warren of Massachusetts speaks at the rally while a protester holds the flag of El Salvador in the foreground. Hide Caption 9 of 16 Photos: Immigration protests around the country New York: Activists shout at the rally to end family separation. Hide Caption 10 of 16 Photos: Immigration protests around the country Casper, Wyoming: People write letters to members of the Wyoming congressional delegation following an immigration rally in Pioneer Park. Hide Caption 11 of 16 Photos: Immigration protests around the country Washington: Protesters, many of them holding signs, gather in Lafayette Square across from the White House. The sign in the foreground translates as "families belong together." Hide Caption 12 of 16 Photos: Immigration protests around the country New York: A protester marches across the Brooklyn Bridge. Hide Caption 13 of 16 Photos: Immigration protests around the country Austin, Texas: A demonstrator uses baby clothes to spell out the word "reunite" during a rally against the Trump administration's immigration policies outside the Texas Capitol. Demonstrations are being held in cities across the United States on Saturday to call for the reunification of separated families and to protest the detention of children and families seeking asylum at the border. Hide Caption 14 of 16 Photos: Immigration protests around the country Albuquerque, New Mexico: A protester holds a sign as demonstrators gather downtown at Civic Plaza calling for an end to family separation. Hide Caption 15 of 16 Photos: Immigration protests around the country New York: Demonstrators at Foley Square raise their fists in protest. Hide Caption 16 of 16

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.