EL PASO, TX - JULY 24:  A man, identified only as Tomas, holds hands with his daughter, Yessica, 13, as they are cared for in an Annunciation House facility after they were reunited yesterday on July 24, 2018 in El Paso, Texas. Tomas and Yessica, originally from Guatemala, were reunitd at an I.C.E processing center two months after the two were separated when they tried to cross into the United States. It is unclear if a court-ordered July 26 deadline will be met for the U.S. government to reunite as many as 2,551 migrant children ages 5 to 17 that had been separated from their families.  (Photo by Joe Raedle/Getty Images)
Joe Raedle/Getty Images North America/Getty Images
EL PASO, TX - JULY 24: A man, identified only as Tomas, holds hands with his daughter, Yessica, 13, as they are cared for in an Annunciation House facility after they were reunited yesterday on July 24, 2018 in El Paso, Texas. Tomas and Yessica, originally from Guatemala, were reunitd at an I.C.E processing center two months after the two were separated when they tried to cross into the United States. It is unclear if a court-ordered July 26 deadline will be met for the U.S. government to reunite as many as 2,551 migrant children ages 5 to 17 that had been separated from their families. (Photo by Joe Raedle/Getty Images)
Now playing
01:58
Families remain separated as deadline passes
CNN
Now playing
01:23
Senator: Family separation is kidnapping
SAN DIEGO, CA - JULY 16:  An American flag flies along a section of the U.S.-Mexico border fence on July 16, 2018 in San Diego, California. The entire Southwest border saw 34,114 U.S. Border Patrol apprehensions in the month of June compared with 40,338 in May.  (Photo by Mario Tama/Getty Images)
Mario Tama/Getty Images North America/Getty Images
SAN DIEGO, CA - JULY 16: An American flag flies along a section of the U.S.-Mexico border fence on July 16, 2018 in San Diego, California. The entire Southwest border saw 34,114 U.S. Border Patrol apprehensions in the month of June compared with 40,338 in May. (Photo by Mario Tama/Getty Images)
Now playing
03:57
Hear: Immigrant moms plead with judge for kids
U.S. Border Patrol agents take a father and son from Honduras into custody near the U.S.-Mexico border on June 12, 2018 near Mission, Texas. The asylum seekers were then sent to a U.S. Customs and Border Protection (CBP) processing center for possible separation. U.S. border authorities are executing the Trump administration's zero tolerance policy towards undocumented immigrants. U.S. Attorney General Jeff Sessions also said that domestic and gang violence in immigrants' country of origin would no longer qualify them for political-asylum status. John Moore/Getty Images
John Moore/Getty Images
U.S. Border Patrol agents take a father and son from Honduras into custody near the U.S.-Mexico border on June 12, 2018 near Mission, Texas. The asylum seekers were then sent to a U.S. Customs and Border Protection (CBP) processing center for possible separation. U.S. border authorities are executing the Trump administration's zero tolerance policy towards undocumented immigrants. U.S. Attorney General Jeff Sessions also said that domestic and gang violence in immigrants' country of origin would no longer qualify them for political-asylum status. John Moore/Getty Images
Now playing
01:26
Parents of 71 children still unidentified
Now playing
01:48
You heard her cry for her mom. See their reunion
CNN
Now playing
01:34
Trump official: Separations are saving lives
Emanuella Grinberg/CNN
Now playing
03:16
Watch daughter wait for separated mom's arrival
U.S. Border Patrol agents take a father and son from Honduras into custody near the U.S.-Mexico border on June 12, 2018 near Mission, Texas. The asylum seekers were then sent to a U.S. Customs and Border Protection (CBP) processing center for possible separation. U.S. border authorities are executing the Trump administration's zero tolerance policy towards undocumented immigrants. U.S. Attorney General Jeff Sessions also said that domestic and gang violence in immigrants' country of origin would no longer qualify them for political-asylum status. John Moore/Getty Images
John Moore/Getty Images
U.S. Border Patrol agents take a father and son from Honduras into custody near the U.S.-Mexico border on June 12, 2018 near Mission, Texas. The asylum seekers were then sent to a U.S. Customs and Border Protection (CBP) processing center for possible separation. U.S. border authorities are executing the Trump administration's zero tolerance policy towards undocumented immigrants. U.S. Attorney General Jeff Sessions also said that domestic and gang violence in immigrants' country of origin would no longer qualify them for political-asylum status. John Moore/Getty Images
Now playing
00:57
Most toddlers still separated from families
Young migrant children, whose faces can not be shown, are seen at the US Customs and Border Protection Facility in Tucson, Arizona during a visit by US First Lady Melania Trump, June 28, 2018. (Photo by MANDEL NGAN / AFP)        (Photo credit should read MANDEL NGAN/AFP/Getty Images)
MANDEL NGAN/AFP/AFP/Getty Images
Young migrant children, whose faces can not be shown, are seen at the US Customs and Border Protection Facility in Tucson, Arizona during a visit by US First Lady Melania Trump, June 28, 2018. (Photo by MANDEL NGAN / AFP) (Photo credit should read MANDEL NGAN/AFP/Getty Images)
Now playing
01:44
Only 54 children to be reunited by deadline
BROWNSVILLE, TX - JUNE 22:  A crying  Honduran woman and her child wait along the border bridge after being denied into the Texas city of Brownsville which has become dependent on the daily crossing into and out of Mexico on June 22, 2018 in Brownsville, Texas. Immigration has once again been put in the spotlight as Democrats and Republicans spar over the detention of children and families seeking asylum at the border. Before President Donald Trump signed an executive order Wednesday that halts the practice of separating families who were seeking asylum, over 2,300 immigrant children had been separated from their parents in the  zero-tolerance policy for border crossers.  (Photo by Spencer Platt/Getty Images)
Spencer Platt/Getty Images North America/Getty Images
BROWNSVILLE, TX - JUNE 22: A crying Honduran woman and her child wait along the border bridge after being denied into the Texas city of Brownsville which has become dependent on the daily crossing into and out of Mexico on June 22, 2018 in Brownsville, Texas. Immigration has once again been put in the spotlight as Democrats and Republicans spar over the detention of children and families seeking asylum at the border. Before President Donald Trump signed an executive order Wednesday that halts the practice of separating families who were seeking asylum, over 2,300 immigrant children had been separated from their parents in the zero-tolerance policy for border crossers. (Photo by Spencer Platt/Getty Images)
Now playing
02:41
Hear emotional phone calls from parents separated from their kids
mother daughter reunion
mother daughter reunion
Now playing
02:12
Watch a mother and child reunite after 55 days
BROWNSVILLE, TX - JUNE 22:  A crying  Honduran woman and her child wait along the border bridge after being denied into the Texas city of Brownsville which has become dependent on the daily crossing into and out of Mexico on June 22, 2018 in Brownsville, Texas. Immigration has once again been put in the spotlight as Democrats and Republicans spar over the detention of children and families seeking asylum at the border. Before President Donald Trump signed an executive order Wednesday that halts the practice of separating families who were seeking asylum, over 2,300 immigrant children had been separated from their parents in the  zero-tolerance policy for border crossers.  (Photo by Spencer Platt/Getty Images)
Spencer Platt/Getty Images North America/Getty Images
BROWNSVILLE, TX - JUNE 22: A crying Honduran woman and her child wait along the border bridge after being denied into the Texas city of Brownsville which has become dependent on the daily crossing into and out of Mexico on June 22, 2018 in Brownsville, Texas. Immigration has once again been put in the spotlight as Democrats and Republicans spar over the detention of children and families seeking asylum at the border. Before President Donald Trump signed an executive order Wednesday that halts the practice of separating families who were seeking asylum, over 2,300 immigrant children had been separated from their parents in the zero-tolerance policy for border crossers. (Photo by Spencer Platt/Getty Images)
Now playing
02:02
HHS won't give exact number of kids in custody
CNN
Now playing
02:13
Berman: DNA test is de facto admission of no plan
CNN
Now playing
02:39
Doctor describes trauma of migrant children
CNN
Now playing
00:52
Mother reunited with her son after 32 days
CNN
Now playing
01:45
Immigration attorney represented 3-year-old
(CNN) —  

Immigrant advocacy groups are gearing up to track down more than 400 parents who were separated from their children and deported without them.

The challenge is so daunting that government attorneys have bristled every time it’s come up in court.

And now a number of nonprofit groups say they’re prepared to join in a massive search.

“It’s going to be really hard detective work,” said Lee Gelernt, an American Civil Liberties Union attorney who’s been leading the organization’s lawsuit over separated families.

Already, some devastated parents are coming forward, desperate to be reunited with their kids, according to advocacy groups. But there are hundreds whose whereabouts are unknown.

Here’s a look at some of the key issues at play – and major hurdles complicating the process:

The government has said parents deported without their kids consented to that. Advocates aren’t so sure.

According to the latest government statistics, some 430 parents from separated families were likely deported without their kids. Officials maintain that before that happened, the parents consented.

“Parents that did return home without their child did so after being provided an opportunity to have that child accompany them on the way home,” said Matthew Albence, who heads US Immigration and Customs Enforcement’s removal operations.

“We cannot force a parent to take a child with them,” he added.

It’s true that some parents do make the harrowing choice to be deported without their kids; they may decide it’s the best option for their families – and the only way for their children to survive.

But the ACLU and other immigrant rights groups argue that parents were coerced into signing paperwork they didn’t understand.

“It is our position that all of those families need to be considered as families that should be reunified and that the government should reunify them,” said Michelle Brané, director of migrant rights and justice for the Women’s Refugee Commission.

That’s a point of dispute that’s likely to come up when ACLU and government attorneys appear in court again on Friday. No matter what officials say, immigrant advocacy organizations say they’re determined to track down all the deported parents to verify details of their cases.

“Our hope is to find each one of those (parents) and find exactly what happened and why are they not with their child and do they want that child back,” the ACLU’s Gelernt said.

Some parents are likely living in remote areas hard to reach. And some might be afraid to come forward.

Finding deported parents will be much easier said than done. Gelernt described it as an “enormous task.”

Why?

A number of factors could make parents afraid to come forward, says Lisa Frydman, vice president of regional and policy initiatives for Kids in Need of Defense. The organization announced Thursday it was launching a new initiative to help reunite children with parents who were deported. The project will involve outreach efforts aimed at finding hundreds of parents in Central America, including working with local governments and community groups. But Frydman said the organization is still pinpointing what steps to take.