Washington (CNN) Migrant families have sent approximately 135 children across the US-Mexico border alone after waiting in squalid and unsafe conditions in Mexico as a result of a Trump administration policy.

The US Department of Health and Human Services told CNN it has identified approximately 135 children in their custody who previously arrived at the southern border with their family and had been returned to Mexico.

In recent weeks, some case managers who work with unaccompanied children began hearing stories from kids who were in Mexico with family members and have since crossed alone, according to a source close to the situation.

A four-year old girl recalled her grandmother sitting her down and explaining she had to cross the border alone, according to a source familiar with these cases. Case managers, who are employed by shelters that work with the Health and Human Services Department, were eventually able to connect with the grandmother in Mexico who explained she worried for her and her granddaughter's safety.

In another instance, an 8-year-old boy crossed the border alone this month after roughly two months in Mexico with his father.

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