The death of an 8-year-old migrant girl in 2023 while she was in the custody of U.S. Customs and Border Protection prompted investigations and the removal of the agency’s chief medical officer. Now, two immigrant rights groups are seeking $15 million in damages on behalf of the girl’s family.

In a wrongful death claim filed with the federal government on Thursday, lawyers for the family offer the most detailed public account yet of the life and death of the child, Anadith Danay Reyes Álvarez, and her family’s efforts to obtain answers about her care in federal custody.

Her death came during a record increase in migration, as the Biden administration struggled to curb illegal crossings and faced criticism about overcrowded detention facilities and the treatment of minors. Illegal crossings plunged in the final months of the Biden administration after a change in asylum policy, and have remained very low under President Trump. But the Trump administration has made families with children targets for detention and removal as President Trump seeks to fulfill a campaign pledge to deport millions of undocumented immigrants.

Rochelle Garza, president of the Texas Civil Rights Project, one of the groups that filed the claim, said Anadith’s family wanted to ensure there was accountability and transparency in Customs and Border Protection facilities, which she described as “one of the most obscure and opaque types of detention in our American immigration system.”

“They do not want their daughter to have died in vain,” Ms. Garza said.

Customs and Border Protection, Immigration and Customs Enforcement and the Department of Homeland Security did not respond to requests for comment on the wrongful death claim. After Anadith’s death, Troy Miller, then acting head of the border agency, requested a review of CBP facilities and made recommendations to address the medical care issues.

Anadith, a Panamanian national, was diagnosed with sickle cell disease and a heart condition at a young age. When she was 5, she traveled with her father to Spain for open-heart surgery and returned to Panama. The family made their way up through Mexico and sought to cross into the United States in May 2023 in hopes of providing safety and a better life for their daughter, according to the complaint.

Her parents, who are Honduran, are members of a long-persecuted Afro-Indigenous population known as Garifuna, and had fled their own country before their daughter was born. The other immigrant rights group that filed the family’s legal claim was the Haitian Bridge Alliance, which focuses on serving Black immigrants.

On May 9, 2023, she, her parents and two siblings were detained alongside other migrants at the border near Brownsville, Texas. The family was then taken to a processing center in Donna, a nearby city, where security camera footage showed her parents handed over their daughter’s medical records to border officials in a medical screening area, the claim states. But medical personnel there did not properly assess her medical history or communicate the details of her medical conditions to the staff at the facility in Harlingen where the family ended up, investigators have found.

Anadith and her family were held in custody for nine days, more than twice as long as newly arrived migrants, particularly children, should be detained, according to the border agency’s own standards. In that time, Anadith exhibited a high fever and complained of pain in her chest and abdomen, among other symptoms, lawyers said.

The claim contends immigration officials failed to provide the girl with proper medical care and to adhere to a 22-year-old consent decree that lays out the minimum standards for care of the nation’s youngest new arrivals. Between the evening of May 14 and her death on May 17, an internal investigation found, medical professionals at the holding facility in Harlingen saw Anadith at least nine times. A nurse practitioner who saw the child told internal investigators that she dismissed three or four requests from Anadith’s mother to call an ambulance or take the child to the hospital.

The previous year, a report from the detention ombudsman at Homeland Security had warned that critical shortages in medical services at border facilities could put migrants’ lives at risk. A report from the Senate Judiciary Committee earlier this year found the circumstances of Anadith’s death were “not an aberration, but indicative of systemic problems” within border facilities and medical care.

Anadith’s family is now in the process of seeking asylum, and her parents have secured work permits, lawyers said.

In an interview on Thursday, the girl’s mother, Mabel Álvarez, said her family had filed the claim in hopes of preserving Anadith’s memory and preventing another tragedy. She recalled that her daughter was healthy when she first arrived at the South Texas border. But she said the small room where her family was detained was filthy with trash and dust. She also recalled it was frigid, the reason such facilities are often referred to as “hieleras,” or coolers.

Ms. Álvarez wept as she described staff members who she said ignored her pleas for medical attention as her daughter’s condition worsened. After the family’s release from immigration detention, Ms. Álvarez said, she took on a job at a factory in New York, but she had to leave it as she struggled with depression and anxiety.

“It was a difficult thing, that my daughter died in my arms, looking for help,” she said.