An investigation has been launched after a seven-year-old girl died while in the custody of the US Border Patrol.
The Guatemalan child, named as Jakelin Caal Maquin, was detained last week after crossing the US-Mexico border with her father, officials say.
It was earlier reported that she died of dehydration, but border officials insist the pair had access to food and water while they were in detention.
A government watchdog will investigate before releasing a final report.
The death of the girl has bought renewed focus on President Donald Trump's hard-line immigration policy, and onto the migrants who are travelling from Central America to the US border.
The migrants say they are fleeing persecution, poverty and violence in their home countries. Many say they are aiming to settle in the US despite warnings that anyone found entering the country illegally will face deportation.
What do the US authorities say happened?
According to US Customs and Border Protection (CBP), the girl was apprehended with her father for illegally entering the country on the evening of 6 December.
She was then screened and found to have no health issues.
It says she was held in a location that had food, water and toilets before she was loaded onto a bus with her father ahead of a 94-mile (151km) journey to the nearest Border Patrol Station.
But the girl began vomiting while on the bus, officials say, and later stopped breathing.
When the bus arrived at the Border Patrol Station she received emergency medical attention and was revived twice before being flown to hospital in El Paso, according to the CBP.
It says she died there after suffering a cardiac arrest and was diagnosed with brain swelling and liver failure.
"Border Patrol Agents… did everything in their power to provide emergency medical assistance," CBP Commissioner Kevin K. McAleenan said in a statement.
"We welcome the Department of Homeland Security's investigation and will review the incident operationally to learn from this tragedy," he added.
Some Democrats have called for resignations over the incident, while others, including former secretary of state Hillary Clinton, have said it is part of a "humanitarian crisis" on the border.
Why is there tension on the border?
It's been running high since the arrival of almost 7,500 migrants in recent weeks.
Last month, US border agents used tear gas on a crowd of migrants, including children, trying to cross the border.
The agents said that personnel had been assaulted and hit by stones.
However, critics accused the Trump administration of a draconian response, while Mexico demanded an investigation into the incident.
The migrants have travelled in large groups, dubbed "caravans", for more than 4,000km from Central America.
Among them are many families with young children.
Donald Trump has vowed to keep each migrant on the Mexican side of the border until courts have decided their cases, meaning some face a long wait.
They have been spending time in temporary shelters in the Mexican border city of Tijuana and in Mexicali, 180km to the east.