Originally Published: 03 MAY 21 05:00 ET

Updated: 03 MAY 21 11:59 ET

By Priscilla Alvarez, CNN

(CNN) -- Four migrant families separated at the US-Mexico border under the Trump administration will be reunited this week, the Department of Homeland Security announced Monday, marking the first reunifications under President Joe Biden.

The effort stems from the family reunification task force set up by one of Biden's executive orders. The task force, housed in DHS, involves federal agencies to identify and reunite families who had been separated at the US-Mexico border under then-President Donald Trump's "zero tolerance" immigration policy.

"The first families reuniting this week are mothers, they are sons, they are daughters, they are children who were 3 years old at the time of separation. They are teenagers who have had to live without their parent during their most formative years," Homeland Security Secretary Alejandro Mayorkas told reporters Sunday.

While Mayorkas declined to provide specific details about the families to protect their privacy, he described two cases -- one mother from Honduras who was separated from her children in late 2017 and another mother from Mexico who was separated from her son in late 2017. Both mothers will be reunited with their children in the coming two days.

Families will be receiving humanitarian parole to enter the United States and reunify with their children who are in the country, according to Michelle Brané, executive director of the family reunification task force.

"The way these families are being returned is night and day from when they were encountered," said Ann Garcia, an attorney who's been working on assisting separated families with legal and social needs. "They are quite literally rolling out the red carpet for these families as they should be for them and every other migrant that comes here."

The task force has been engaging with groups who are in touch with families and carefully planning their return, taking into account past trauma, Garcia said, noting that some parents are nervous to encounter US Customs and Border Protection again since their last encounter with agents resulted in their kids being taken from them.

The families are among the thousands who were separated under the Trump administration's controversial "zero tolerance" policy, which resulted in the separations of at least 2,800 children from their parents, according to government data. Officials later found more than 1,000 children had been separated from their families before Trump's policy went into effect in 2018.

Brané said the task force has identified more than a thousand families that remain separated, or that are believed to still be separated.

Attorneys in an ongoing family separation case, meanwhile, are still trying to reach the parents of 445 migrant children torn apart from their families at the US-Mexico border between 2017 and 2018.

"We are excited that these mothers will finally see their children after years, but the (American Civil Liberties Union) is certainly not prepared to celebrate just yet given the thousands who still need to be reunified and the more than 5500 children who were traumatized and need help," said ACLU attorney Lee Gelernt, the lead lawyer in the ongoing family separation case.

"We are pressing for permanent legal status, compensation and social services. It is the least these families deserve given that our government deliberately abused them," he added.

Mayorkas touted the work of the task force, which has been sifting through thousands of records and correcting issues in existing files.

As part of the effort, DHS is establishing a process for accepting parole requests, the Department of Health and Human Services is working on facilitating services to support families, and the State Department is developing a streamlined system for processing in-country travel document requests. The Justice Department is also involved in related settlement negotiation efforts.

"Since the executive order was signed, the task force has been working to identify parents and children separated by the prior administration, establish a system to reunite them safely, and ensure they are provided support after the traumatic experience they endured," Mayorkas said.

Immigrant advocates, who for years scrambled to find and connect families, have urged the Biden administration to focus on rectifying the damage of the previous administration. Advocate groups have also called for accountability and transparency to fully account for the "zero tolerance" policy and its ramifications, as well as for factoring in the input of parents who were separated from their children in an attempt to restore trust with families harmed by the former administration.