SAN DIEGO (AP) — Immigrant children around the U.S. left shelters with their backpacks and a tender goodbye hug from staff members as the Trump administration began reuniting dozens of youngsters with their parents Tuesday under a court-ordered deadline.

It was the largest single effort to date to undo the effects of President Donald Trump’s zero-tolerance policy of separating families who try to slip across the Mexican border into the country.

More than 50 children under age 5 could be back in the arms of their parents by the deadline at the end of the day, the Justice Department said. Authorities gave few details on where the reunions would be held, and many were expected to take place in private.

Government attorneys returned to federal court in San Diego on Tuesday to seek an extension for releasing 20 other children under 5, saying officials need more time to track down parents who have already been deported or released into the U.S.

Asked about the missed deadline, Trump said: “Well, I have a solution. Tell people not to come to our country illegally. That’s the solution.”

Meanwhile, the administration faces a second, bigger deadline — July 26 — to reunite thousands of older children who were also separated from their families at the border in the past few months.

Staff members at a nonprofit organization that has been housing many of the youngest children “made sure every backpack was full and every child got a hug and a goodbye,” Southwest Key CEO Juan Sanchez said.

Most of the parents will be released into the U.S. from immigration detention centers, and the children will be freed from government-contracted shelters. The adults may be required to wear ankle monitors while their cases wind through immigration court, a process that can take years.

More than 2,000 children were separated from their parents by U.S. immigration authorities at the border this spring before Trump reversed course on June 20 amid an international outcry.

Late last month, U.S. District Judge Dana Sabraw in San Diego set a 14-day deadline to reunite children under 5 with their parents and a 30-day deadline for older children.

In trying to meet the first deadline, the government began with a list of 102 children potentially eligible to be reunited and whittled that to 75 through screening that included DNA testing done by swabbing the inside of the cheek.

The government defended its screening, saying it discovered parents with serious criminal histories and one case of credible child abuse.

“Our process may not be as quick as some would like, but there is no question it is protecting children,” said Chris Meekins, a Health and Human Services Department official helping to direct the process.

In fact, through DNA testing, two adults who apparently thought they were parents of a child were determined not to be, he said.

“If we find out they are not the legal parent, then clearly we are not going to reunite them,” Meekins said.

In ordering an end to the separation of families, the president said they should instead be detained together. But the government does not have the room: Immigration and Customs Enforcement has three family detention centers with space for 3,000 people, and they are already at or near capacity, though the Trump administration is trying to line up space at military bases.

Also, on Monday, a federal judge in Los Angeles emphatically rejected the Trump administration’s efforts to detain immigrant families for an extended period. A longtime court settlement says children who cross the border illegally cannot be detained for more than 20 days.