McALLEN, Texas — The Justice Department on Thursday asked a federal judge to change the rules regarding the detention of immigrant families who enter the country illegally, seeking permission to detain them for longer than 20 days in an effort to keep children with their parents.

Lawyers filed a memorandum to a settlement in California that governs how children are handled when they are caught crossing the U.S. border illegally. The Flores settlement states that families cannot be detained longer than 20 days.

The move is aimed at stopping the separation of children from their families amid a new policy where anyone caught crossing the border is charged criminally.

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The request comes a day after President Trump signed an order stopping the practice that has resulted in the separation of more than 2,300 children from their families. The Trump administration's so-called zero tolerance policy will remain in place, but families will be kept together in detention.

Despite Mr. Trump's executive order, a host of unanswered questions remain, including what will happens to the children who were already separated from their parents and where the government will house all the newly detained migrants, with the system already bursting at the seams.

In the Texas border city of McAllen on Thursday, prosecutors unexpectedly dropped charges against 17 immigrants who were due to be sentenced Thursday for improperly entering the U.S., according to Efren Olivares, a lawyer with the Texas Civil Rights Project, which has been interviewing adult immigrants to track them and their kids through separate government systems.

Speaking outside of the federal courthouse, Olivares said the 17 likely would be placed in immigration detention. He didn't know whether they would be reunited with their children immediately or whether they might be released altogether.

Asked if they had any reaction to the charges against them being dropped, he said, "They're asking about their children, frankly."

Meanwhile, three of the 12 immigrant children at a Catholic Charities shelter in Fort Worth, near Dallas, were expected to be reunited with their family on Thursday.

Heather Reynolds, the nonprofit's director, says the three are among 12 immigrant children at the shelter who were separated from their parents under a Trump administration "zero tolerance" policy. She says half are boys and half are girls, and they range in age from age 5 to 12.

Reynolds declined to provide details about the three kids who are expected to be reunited with family Thursday.

She says Mr. Trump's executive order Wednesday allowing immigrant kids to remain with their parents mentions how future illegal border crossing will be handled, but it doesn't address the more than 2,000 children who have already been separated since May. She says this leaves groups like hers "uncertain" about how to manage those kids already in detention in the U.S.

First lady Melania Trump made a surprise visit Thursday to a McAllen detention center that's housing some of the children, where she asked kids where they came from and how long they had been at the facility.

She told children to "be kind and nice to each other" as she left one classroom and said she wanted to lend her support to the children, and asked staff to reunite them with their families as quickly as possible.

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Montoya Bryan reported from El Paso.