Washington (CNN) The whereabouts of thousands of migrants waiting in Mexico for their upcoming court date in the United States is unknown, according to attorney accounts and data analyzed by CNN.

Nearly 55,000 migrants, many of whom are from Central America, have been sent back to Mexico as part of a Trump administration policy that requires them to wait there until their court date in the United States.

An individual's presence at their court date is one of the few accounting mechanisms for the population of people who have fallen under the policy. But as those dates approach, many migrants -- often times, waiting in perilous conditions in Mexico -- aren't showing up, highlighting in part the untenable conditions along the southern border.

Half of the people waiting in Mexico for a scheduled court date were not present at their last hearing, according to data from Syracuse University's Transactional Records Access Clearinghouse, which tracks immigration court data.

To compare, nearly 90 percent of immigrants who are in the US attended their court hearing, TRAC found.

An immigration attorney in Texas, Norma Sepulveda, recalled the case of a young migrant woman who waited in northern Mexico for around three months before her first court hearing in the United States. During that time, she was sexually assaulted twice, Sepulveda said.

"She was so afraid that whenever these men would walk in the door, she'd urinate on herself out of fear and she just couldn't handle it anymore," Sepulveda, who represented the woman, told CNN, referring to one of the shelters in which the woman stayed.

The woman, driven by fear, eventually moved further into the interior of Mexico and was unable to return to the US for a follow-up court hearing. Her absence will now largely bar her from being able to claim asylum in the US in the future.

She is one of nearly 17,136 people told to wait in Mexico but did not appear at their last scheduled hearing, according to TRAC

The reasons for why someone might not return to the US for their court hearing run the gamut. Some people might not have a way of getting to their respective entry point to then be transported to court in the US; some have been kidnapped or assaulted while waiting in Mexico; and others have chosen to return to their native country after experiencing difficult conditions in encampments, according to interviews with attorneys and immigrant advocates.

When asked for comment on TRAC's figures, a spokesperson for the Justice Department's Executive Office for Immigration Review, which oversees the nation's immigration courts, said it could not comment on what happens to individuals.

"The dataset we make available to the public in a raw dataset form is provided as a resource and is not something that our office can assist with for reporting," the Justice Department said.

The Trump administration implemented the so-called "remain in Mexico" policy amid an increase in migrant arrivals at the southern border, arguing that it would deter migrants from making the journey north.

Despite ongoing legal challenges, a federal appeals court allowed the policy to continue . As a result, the policy has expanded across the southern border, subjecting more asylum seekers to the policy and in many cases, returning them to dangerous conditions.

Migrants face obstacles in getting to their hearings in the US

The Trump administration had said that it planned to expedite hearings for migrants who fell under the policy. But even so, it can take weeks, if not months, for an individual's case to be resolved. And in the interim, migrants are often waiting in squalid and unsafe conditions in Mexico.

Some of the regions where migrants are left waiting have been deemed dangerous by the US State Department.

Human Rights First, an advocacy organization, has identified violent attacks against migrants who've been returned to Mexico in regular reports. In an October report, the group found more than 340 public reports of rape, kidnapping and torture, among other attacks against migrants waiting in Mexico.

In testimony before a House panel last month , Erin Thorn Vela, a staff attorney at the Texas Civil Rights Project who works with migrants in Matamoros, Mexico, recalled the kidnapping of a mother and her child.

"One mother and her small child were kidnapped less than one hour after the US government forcibly returned them to Matamoros," Thorn Vela told lawmakers. "They were tortured for 8 days."

Acting Customs and Border Protection Commissioner Mark Morgan has defended the policy despite the dangerous conditions.

"We are continuing to work with the government of Mexico. We provided them millions and millions of dollars to help with their humanitarian push that's there," Morgan told reporters earlier this month.

The administration offers a follow-up interview for migrants who say they're afraid of returning to Mexico, but only a small share have met the standard, according to CBP data obtained by CNN.

Still, the insecurity and at times, unsanitary conditions, have led some migrant families to send their children across the US-Mexico border alone. The US Department of Health and Human Services told CNN late last month that it had identified approximately 135 children in their custody who previously arrived at the southern border with their family but are now in the US alone.

"The people that left did put some thought into why they're leaving Central America, they have the ability to determine what the situation is they're in," said Kelly Overton, founder of Border Kindness, which helps transport migrants in Mexico to their court dates in the US. "In the end, can they get somewhere here where they can feed their kids and their kids are safe while they wait?"

'You literally just can't wait it out'

Border Kindness has made more than 2,500 trips transporting more than 1,100 people from Mexicali to Tijuana for their immigration court in San Diego. Migrants report to US officials at the border on the day of their hearing, where they are transported to a courtroom.

But cases aren't resolved in one hearing. There are follow-up hearings to argue the merits of the case and the time it takes between hearings can wear down on migrants. By the second or third trip back to Tijuana, more and more people stop showing up, deciding to either return to their home country or move to another part of Mexico, Overton said.

"You literally just can't wait it out," Overton added. "You don't see an end in sight."

Immigration lawyers have warned that in some cases, documents provided to migrants sometimes contain incorrect hearing locations and addresses and incorrect court dates.

The Justice Department told CNN that "immigration courts nationwide may receive defective filings for a variety of reasons," and directed questions regarding filings to the Department of Homeland Security, which has not responded to a request for comment.

Attorneys have also faced their own set of hurdles when trying to communicate with clients that aren't located in the US. Sepulveda noted the difficulty in communicating with the young woman she represented who didn't have a phone of her own.

Lawyers who have crossed over to Mexico to help clients have had to take into consideration their own safety. Laura Peña, pro bono counsel for the American Bar Association Commission on Immigration, recalled during a House hearing in November taking precautions when traveling to Mexico to visit clients, including crossing only during the day and coordinating with humanitarian groups or other colleagues in the area.

If not for the policy, her clients would likely be in the US where they'd receive counsel in an attorney's office or other private location.

Not all migrants have an attorney, however -- often times, putting them at a disadvantage. Research has underscored the significance of access to counsel for immigrants. Research has shown that immigrants with lawyers are more likely to be allowed into the US.

Concerns over transparency in the immigration process

In expanding the so-called "remain in Mexico" policy earlier this year, the Trump administration built tents in Laredo and Brownsville, Texas, to serve as makeshift courts for migrants waiting in Mexico until their court date. The judges in these cases preside by teleconference from other immigration courts several miles away.

US officials told CNN in June the temporary courts would allow migrants to have their hearings near or at the port, rather than being bussed miles away.

But advocates, immigration lawyers, and former immigration judges have raised alarm over the use of the tent facilities, arguing that it shrouds the court proceeding in secrecy, since the public isn't allowed in.

Those concerns were exacerbated when the administration said it would begin using one of its adjudication centers to hear cases. Up until now, legal observers have been able to watch proceedings happening at the tent facilities from the court where the judge is based. But adjudication centers, which serve as a hub for immigration judges to beam into cases remotely, are not open to the public.

More than a dozen former immigration judges recently sent a letter to Executive Office for Immigration Review Director James McHenry underscoring in part the importance of public access to immigration courts, calling it "vital to the constitutional protections of the respondents who appear in court."