Four migrant parents separated from their children at the U.S. border under the Trump administration and deported alone will be allowed to return to the United States this week, Homeland Security Secretary Alejandro Mayorkas announced Monday.

The reunions will mark the beginning of a process that will stretch on for months and possibly years, as separated parents are ferried back to the United States from around the world.

More than a thousand families remain apart, according to the Department of Homeland Security. The parents were deported alone, mostly to Central America, in 2017 or 2018. Their children have since grown up with relatives across the United States.

Some of those children were so young when they were taken from their parents that they barely remember their mothers or fathers. Others have forgotten the indigenous dialects with which they once spoke to their parents.

“Today is just the beginning,” Mayorkas said in a statement released in the early hours of the day. “We are reuniting the first group of families, many more will follow, and we recognize the importance of providing these families with the stability and resources they need to heal.”

As a candidate, Joe Biden promised to reunite those parents and children, calling the Trump administration’s policy of family separation “criminal.”

Speaking to reporters ahead of the announcement, Mayorkas said that four families would be reunited. Two of them include mothers who were separated from their children in late 2017, one Honduran and the other Mexican, Mayorkas said.

Some of the children who will be reunited with their parents this week were as young as 3 when they were separated at the border.

The Trump administration formally implemented its policy of “zero tolerance” from April to June of 2018, when a federal judge ordered an end to the policy and demanded that separated families be reunited.

But it later emerged that the Trump administration had been regularly separating families through much of 2017, and that many of those parents had already been deported without their children. Government documents now show that more than 5,500 children were separated from their parents in 2017 and 2018.

The government had failed to collect or keep contact information for many of the families it had separated. In many cases, parents effectively vanished after being deported without their children.

Some parents were given the choice to reunite with their children back in their home countries, and weighed that option against the threats the children would face if they returned. Many of those parents tried to keep ties with their children over video calls.

“I’m not sure how much he remembers me,” said one of the mothers who will be reunited her child this week, a 26-year-old Honduran woman whose son, now 6, is in Texas.

She has not yet told her son that she will be returning to the United States this week, worried that something could still go wrong and that his hopes could be dashed.

“I’m so, so happy,” she said, “but I’m also nervous.”

Mayorkas, who is leading the government’s Family Reunification Task Force, said the group has been “working day and night, across the federal government and with counsel for the families and our foreign partners, to address the prior administration’s cruel separation of children from their parents.”

But the process has not been simple. For months it remained unclear what kind of legal status returning parents would receive, or how they would travel from remote villages in Central America to the United States. Who would apply for their passports? Could they bring other family members with them?

Even finding some of the separated parents proved difficult. Lawyers representing the separated families still have not located 465 parents, many of whom were probably deported alone to Central America. Searches continue for those parents across the region.

The Biden administration worked with a group of about 10 lawyers and advocates to select 36 families from across Mexico and Central America, who will be reunited with their children in the United States over the next several weeks. Those families are part of a trial run, meant to prepare the federal government for hundreds of future reunifications, an effort unlike anything the U.S. has done before.

Over the last few weeks, members of the 10-person group began calling a handful of separated parents. Were they ready to be with their children in the United States?

Some of the parents were in disbelief. Ann Garcia, a staff attorney with the Catholic Legal Immigration Network and a member of the group, remembers her call with one Guatemalan mother.

“She said, ‘Why did you choose me?’ When we explained, she said ‘I’m grateful to you, the government, and God for choosing me.'”

Lawyers and advocates have been working with the government to guarantee legal status for returning parents, so they can’t be re-separated from their children.

“We’re looking at the tip of the iceberg,” said Lee Gelernt, the lead attorney in a class-action lawsuit that the American Civil Liberties Union brought against the policy in 2018.

“The really hard part isn’t bringing in these four families, but sticking through the whole process. We need to make sure these families get permanent status, compensation, and social services if they have any chance of succeeding.”

Gelernt is hoping the settlement of the lawsuit will include a commitment from the U.S. government to fund the hundreds of pending family reunifications, possibly contracting with nongovernmental organizations to carry out the logistics, including transportation.

This week’s reunifications have been more ad hoc, with attorneys and advocates paying for flights through nonprofits like Miles4Migrants.