The American Civil Liberties Union said in a court filing submitted on Tuesday that they have been unable to contact parents of 545 children who were separated at the border by the Trump administration, leaving the children living with sponsors throughout the United States.
"We haven't found all the families," Lee Gelernt of the ACLU told ABC News' Chief National Affairs Correspondent Tom Llamas. "We are still searching for approximately 545 families that we haven't reached the parent."
Soon after taking office, President Donald Trump imposed a crackdown on families seeking asylum at the U.S.-Mexico border that ultimately led to thousands of family separations. Children were placed in shelters, often unaware of what was happening to their parents, who were detained and likely deported.
While the government eventually placed the children with sponsors, a federal judge ordered the government to track the whereabouts of the parents -- a difficult task because the government failed to adequately track the families in the first place, according to a government watchdog office.
Gelernt said that close to 5,500 families were separated.
"Each one of these children is its own story," Gelernt said. "And those stories are absolutely heartbreaking."
"One 4-year-old from Honduras had glasses," he continued, telling the story of one family. "The boy's parents had been able to get him a case to protect the glasses." Gelernt said the "glasses case became the most important thing in their life because they knew if the glasses broke, they might not be able to get him another pair."
When the boy was separated from his mother, he had his glasses, but he didn't have the case.
"All day long, all the mother thought about was, 'Can my little boy see?'" Gelernt said.
The ACLU and other groups sued the Trump administration over the practice of separating families. The court filing is a part of the ongoing effort to reunite the families that were separated. But the reunification process has often been unsuccessful due to the government's poor tracking system.
A court-appointed "steering committee" has tried to locate the families of 1,030 children. Court documents estimate that approximately two-thirds of the parents are believed to be in their "respective countries of origin."
But Chase Jennings, a Department of Homeland Security spokesperson, said DHS, Customs and Border Protection, Immigration and Customs Enforcement and the Department of Health and Human Services have "taken every step to facilitate the reunification of these families where parents wanted such reunification to occur."
"The simple fact is this: after contact has been made with the parents to reunite them with their children, many parents have refused," Jennings said in a statement provided to ABC News. "In the current litigation, for example, out of the parents of 485 children whom Plaintiffs' counsel has been able to contact, they have yet to identify a single family that wants their child reunited with them in their country of origin. The result is that the children remain in the U.S. while the parents remain in their home country."
In line with DHS, White House deputy press secretary Brian Morgenstern said the administration has "done everything we can to bring these families together."
"It's very sad, the administration wants the families to be reunited, but for various reasons the families just have not accepted the children back in many of these cases," Morgenstern said.
In response, Gelernt referred back to the court document that says that the steering committee has "not yet reached the separated parents" of the 545 children.
"We have not even found these 545 parents so neither we nor certainly the administration can know whether they want to be reunited," Gelernt told ABC News.
Ahead of Thursday's presidential debate, Vice President Joe Biden released a statement calling the reports "an outrage, a moral failing, and a stain on our national character," adding that the principle that "families belong together" will be at the "core" of his immigration policy if he is elected in November.
Biden has pledged to create a path to citizenship for the 11 million undocumented individuals living in the United States.
The judge ruling over the case has scheduled a hearing for Thursday.
"The child doesn't know whether they'll ever see their parent again," Gelernt said, adding that the separation of these families is both historic and can never be forgotten.
"This can never be repeated again."
ABC News' Johnny Verhovek, Bonnie McLean, Victoria Moll Ramirez, Anne Flaherty and Luke Barr contributed to this report.