By law, all minor detainees in the United States MUST be remanded out of custody to somebody else within 20 days. This has been the law in the United States for approximately the last 21 years.
There was a settlement in 1997 called the Flores Settlement. It happened under the Bill Clinton administration. In that settlement, it said that unaccompanied minors (i.e. children where there are no parents present ) in the United States cannot be held in detainment facilities. Instead, the child has to be released to foster care or to a relative who can be tracked down for follow-up. It did not apply to accompanied minors. So if a parent came with their child, then there is no legal obligation under the 1997 Flores Settlement to separate the child.
This changed in 2008 with a piece of legislation (Reauthorization Act) which was enshrined in 2016 [Flores v. Lynch, No. 15-56434 (9th Cir. 2016) see link below]. The 9th circuit court of appeals Judge Andrew D. Hurwitz wrote an opinion that ruled that the 1997 Flores Settlement did not only apply to unaccompanied minors but also to that accompanied minors.
In a nutshell, this means that all kids based on the 2016 ruling must be separated from their parents. Under the old 1997 law, the child could stay with the parent. Under the new law (2016 ruling), the child would be removed from the parent and remanded to foster care or to a relative.
Here is an exact quote from the ruling:
"The court concluded that the settlement unambiguously applies both to accompanied and unaccompanied minors, but does not create affirmative release rights for parents. The district court erred in interpreting the Settlement to provide release rights to adults."
So what this simply says is that you do not have to release the adult but you do have to release the child. In simpler terms, if you arrest the adult, the adult must stay and the child must go.
Trump was elected the following year.
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References:
1997 Flores Settlement:
https://www.aclu.org/files/pdfs/immigrants/flores_v_meese_agreement.pdf
Reauthorization Act of 2008:
https://www.state.gov/j/tip/laws/113178.htm
2016 Flores v. Lynch, No. 15-56434 (9th Cir. 2016:
https://law.justia.com/cases/federal/appellate-courts/ca9/15-56434/15-56434-2016-07-06.html
Ben Shapiro Podcast: