WASHINGTON — A federal judge on Friday ruled that the Trump administration is responsible for migrant children separated even before it instituted a "zero tolerance" policy.

The ruling followed a report from the Department of Health and Human Services Inspector General that found that "potentially thousands" of children were separated from their parents between June 2017 and May 2018, when the Trump administration began prosecuting under the zero tolerance policy all those crossing the border illegally and separating parents from their children in the process.

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The plaintiffs in the case, led by the American Civil Liberties Union, had argued that these children should be included in the class action lawsuit so that they, like the 2,800 children separated under zero tolerance, could potentially be reunified with their parents.

President Donald Trump talks to reporters as Department of Homeland Security Secretary Kirstjen Nielsen, from right, looks on at a signing ceremony for the Cybersecurity and Infrastructure Security Agency Act in the Oval Office on November 16, 2018. Jonathan Ernst / Reuters file

Lawyers for the Trump administration had argued that the process would be overly burdensome because the vast majority of the children have likely already left government custody.

In his ruling on Friday, Judge Dana Sabraw of the Southern District of California wrote that isn't a sufficient reason to deny the administration's responsibility for the children.

"The hallmark of a civilized society is measured by how it treats its people and those within its borders," Sabraw wrote. "That Defendants may have to change course and undertake additional effort to address these issues does not render modification of the class definition unfair; it only serves to underscore the unquestionable importance of the effort and why it is necessary (and worthwhile)."

For now, the children are only included in the lawsuit, and Sabraw has not ordered that the government reunite them with their families.

Last month, Sabraw said he would first rule on whether these children were included under the class and then he would decide how the government should respond — and if that includes finding them, counting them and potentially reunifying them by a certain deadline.