Washington Post reports the child died of dehydration and shock after being arrested by Border Patrol agents.
A seven-year-old girl has died after being taken into custody by the US Border Patrol, the Department of Homeland Security (DHS) confirmed on Thursday.
On December 6, the child and her father had been held by immigration authorities in the US state of New Mexico as part of a group of 163 people who approached US agents to turn themselves in, the newspaper reported.
Early on December 7, the girl was found to have a 41-degree-Celsius fever and was taken by helicopter to El Paso hospital in Texas where she died, according to the report.
A statement by a DHS spokesperson said: "an accompanied female juvenile detainee began having seizures."
The names of the girl and her father were not released. The agency, which typically provides food and water to migrants in its custody, is investigating the incident to ensure whether appropriate policies were followed, the Post said.
The death of the child is expected to intensify scrutiny over the conditions endured by those held at Border Patrol stations and Customs and Border Protection (CBP) facilities under the administration of Donald Trump, who has made toughening immigration policies a central tenet of his presidency and has pledged to build a wall along the southern border with Mexico.
"This tragedy represents the worst possible outcome when people, including children, are held in inhumane conditions," Cynthia Pompa, advocacy manager for the ACLU Border Rights Center, said in a statement.
"Lack of accountability and a culture of cruelty within CBP have exacerbated policies that lead to migrant deaths," she added. "In 2016, migrant deaths increased even as the number of border crossings dramatically decreased."
Jerry Nadler, the top Democrat on the House of Representatives Judiciary Committee, wrote on Twitter that the head of the Department of Homeland Security, which oversees Customs and Border Protection, will appear in front of the panel next week.
"We will be demanding immediate answers to this tragedy," Nadler said.
Trump has sought to sow fear over thousands of migrants and refugees who have recently arrived at the border as part of an exodus, initially dubbed the Central American caravan. More than 6,000 people are currently waiting in Tijuana, northwestern Mexico, to file for asylum in the US.
Rights groups estimate many will have to wait up to two months before being allowed in the United States to submit their claims.
Many of the refugees and migrants have told Al Jazeera they are fleeing violence, poverty and political persecution.
Trump has sent more than 5,000 troops to the border to offer logistic support to border patrol agents. The Department of Defence approved a plan to extend the deployment of about 4,000 active-duty troops through January.
"When the Trump administration pushed for the militarisation of the border, including more border wall construction, they are driving people fleeing violence into the deadliest desert regions," said ACLU's Pompa, calling for a "rigorous investigation" into the girl's death and "serious reforms to prevent future deaths".
"The fact that it took a week for this to come to light shows the need for transparency for CBP."
This summer, the Trump administration's "zero tolerance" immigration policy, which resulted in separating children arriving at the border with their parents, caused a national outcry. The policy was mostly reversed.