A heartbreaking video shows a young immigrant boy squirming away from his mother after they were reunited.

Under the Trump administration's "zero tolerance" policy, thousands of children were separated from their parents and not all of the reunifications have been easy.

Pediatricians and mental-health experts have been sounding the alarm over the trauma caused to children by such separations, and the potential long-term consequences.

In the video, the young boy's mother can be heard tearfully asking, "What is wrong with my son?"

The Trump administration has reunited most of the roughly 2,600 immigrant families it separated at the height of its "zero tolerance" policy — but the trauma may be far from over for many of the young children who endured months away from their parents.

The American Civil Liberties Union, which has led the charge for the reunification of all separated families, published a heartbreaking video showing one mother reuniting with her three-year-old son. But unlike other footage aired by news outlets showing reunited families hugging and rejoicing, the ACLU video shows the young boy visibly upset by his mother's embrace.

He squirms out of her grip when she wraps her arms around him, then crawls away when she tries to go after him.

"I'm your mommy," she says repeatedly to the boy, as he leans away from her, at one point crawling between his father's legs to get away. "What is wrong with my son?"

"My son is traumatized," she says, her face buried in her shirt.

Watch the full video:

The young boy, whom the ACLU identified only by the pseudonym Sammy, had crossed the US-Mexico border with his father in April 2018, and were separated for more than three months. Later, they reunited with Sammy's mother at the Houston airport.

Sammy's mother is not the only woman whose child didn't respond well to the reunifications. The New York Times reported in July that some of the youngest children who were separated from their parents didn't recognize the mothers who had been waiting months to see them.

Pediatricians and mental-health experts have for months decried the "zero tolerance" policy, fearing the trauma and long-term health consequences that separations could trigger.

Experts have told Business Insider that removing a primary caregiver like a parent from a child — with no indication of when they'll return — can cause acute distress that damages a young child's ability to self-soothe. The stress can even lead to depression and anxiety in the short-term, as well as later in life.

Justice Department attorneys filed court documents this week saying that roughly 500 separated children remain in government custody, and have yet to be reunited with their parents. Twenty-three of those children are under the age of five.

As for Sammy, the ACLU said he eventually allowed his mother to pick him up, and the family left the airport together.