New information provided to a federal court has revealed that the Trump administration separated more "tender age" migrant children than previously known, with the government conceding that an additional 207 minors under the age of 5 years old were separated from their parents before the "zero tolerance" policy was fully implemented.

According to the American Civil Liberties Union (ACLU), the Trump administration on Thursday disclosed an additional 1,556 family separations to the U.S. district court in San Diego, the court overseeing the litigation surrounding the widely denounced practice. The ACLU told CBS News that 207 of the newly identified children who were separated from their parents by the government are under 5 years old — including five infants under the age of 1.

The Trump administration had previously admitted to separating about 107 minors under the age of 5 at the height of the "zero tolerance" policy, in which border-crossing parents were referred for criminal prosecution and forcibly separated from their children. But the new disclosures reveal that the administration separated at least 314 of these "tender age" children, who pediatricians say are particularly vulnerable to separation-induced psychological trauma.

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The ACLU does not know whether the more than 1,500 newly identified families have been reunited or the circumstances under which they were separated by the government. The group has been working for months to help reunite the approximately 2,800 families the Trump administration initially said it separated while it was implementing the "zero tolerance" policy.

"We don't know anything about those families," Lee Gelernt, the attorney leading the ACLU litigation on family separations, told CBS News in an exclusive interview. "We hope that some of those families have even able to reunite through self help mechanisms — but we suspect many, many of them have not."

A migrant toddler is cradled by a Comprehensive Health Services, Inc. caregiver at a "tender-age" facility for babies, children and teens, on Aug. 29, 2019, in San Benito, Texas. Sheltering migrant children has become a booming business for Comprehensive Health Services, a Florida-based government contractor, as the number of children in government custody has swollen to record levels over the past two years. Eric Gay / AP

Although the ACLU is unaware of the exact time period in which the newly disclosed families were separated, they all occurred between July 2017 and June 2018, when Judge Dan Sabraw of the U.S. district court in San Diego barred the administration from separating detained migrant families unless the parents posed a danger to their children. Gelernt said he suspects that the separations were part of pilot programs the administration implemented before fully enforcing "zero tolerance" along the entire border with Mexico.

Since Sabraw's ruling to stop most separations in June 2018, the administration has told his court that it has separated about 1,000 families since, citing concerns that the parents posed a danger to their children and other circumstances strongly contested by the ACLU.

Citing ongoing litigation, the Justice Department, which represents the administration in federal courts, declined to comment on the new revelations.

The disclosure on Thursday contradicts assertions repeatedly made by the administration that it tried not to separate tender age children from their parents. Just this week, former Homeland Security Secretary Kirstjen Nielsen, who became one of the public faces of the family separations saga, said her department looked to enforce the law in the "most compassionate way possible."

The @ACLU to @CBSNews: 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. Ex-DHS Sec. Nielsen told @IAmAmnaNawaz that her agents tried not separate children under 5.https://t.co/Lhi4ALrUhm — Camilo Montoya-Galvez (@camiloreports) October 25, 2019

"If you had children under 5, the policy was we would not refer you to prosecution, despite the fact that you had broken the law," she told PBS NewsHour on Tuesday at a FORTUNE summit before being told by the moderator that "tender age" minors were in fact separated from their parents by Nielsen's agents.