New York — The government told a federal court on Thursday that more than 1,500 migrant families were forcibly separated by U.S. authorities before the Trump administration's "zero tolerance" policy was fully implemented, meaning a total of at least 4,300 families were separated before a judge ordered officials to halt the controversial practice.

Complying with a court-approved demand from the American Civil Liberties Union (ACLU) to account for previously undisclosed family separations, the Trump administration told the U.S. district court in San Diego that it identified an additional 1,556 migrant children that it separated from their parents, according to the ACLU. The group said 207 of the newly identified children who were separated from their parents are under 5 years old, including five infants under the age of 1.

"Those are 1,600 hundred families we'll now have to find and search all over the world for," Lee Gelernt, the lead ACLU attorney in the ongoing litigation surrounding family separations, told CBS News in an interview Thursday.

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The ACLU does not know whether the newly identified 1,556 families have been reunited.

The Justice Department, which represents the administration before federal courts, declined CBS News' request for comment, citing ongoing litigation.

Although some detained migrant families were separated under previous administrations — mostly when officials determined the parents posed a danger to their children — the Trump administration utilized different tools, including the policy of "zero tolerance" that led to the criminal prosecutions of border-crossing parents, to systematically separate thousands of families in the span of months.

After massive public outcry, Judge Dana Sabraw U.S. District of Southern California ordered the administration in June 2018 to halt the controversial practice of forcibly separating detained migrant parents from their children. Sabraw decreed that families should not be separated "absent a determination that the parent is unfit or presents a danger to the child."

According to figures disclosed through litigation, more than 2,800 children were separated from their parents under "zero tolerance" as of late 2018. Approximately 1,000 migrant minors were separated from their parents after Sabraw's ruling. The separations revealed on Thursday had never before been disclosed by the government.

"What's shocking is when we went into court, the government said there is approximately 2,800 families that were separated. We worked for months and months trying to find those families and reunite those families," Gelernt said. "Then we found out only because of an internal watchdog that broke the story that there may have been thousands more families separated ... that the government never told us about, the court or Congress."

"We've finally gotten the list of names from the government," he added.

Although his group has not been informed about why the government separated the newly identified families, Gelernt said the separations could stem from pilot programs the administration ran before fully implementing the "zero tolerance" policy along the entire southern border in the spring of 2018.

Gelernt said the new revelations mean the publicly available figures now suggest that the Trump administration separated approximately 5,500 families at the very least.

"And it's ongoing. That's the really scary part. We get a list from the government every month," he said.