Washington — Carlos Gregorio Hernández Vásquez was in visible pain and disoriented when he collapsed near the toilet in his concrete cell around 1:36 a.m. one day last May. After writhing on the ground for several minutes, the migrant boy from Guatemala laid motionless. Four hours later, he was discovered by his cellmate in the same position, unresponsive and without a pulse.

New footage obtained and published by ProPublica on Thursday shows the agonizing last moments of Hernández Vásquez's time inside the Border Patrol station in south Texas where he died in May, sparking outcry from advocates who pointed to other deaths of migrant children in U.S. custody during the Trump administration. An autopsy later revealed the 16-year-old boy died of the flu and complications from other infections. 

The new video suggests that officials in the facility failed to recognize and address the teenager's distress in the early morning hours as he laid motionless within view of the cell's windows. The recording also contradicts some of the information authorities initially released publicly about his death.

The footage from inside Hernández Vásquez's cell, which ProPublica obtained from the Weslaco, Texas, police department, begins at 1:19 a.m. on May 20. The Guatemalan teenager can be seen coming from the toilet area of the cell, where there is another boy sleeping under a Mylar blanket. At this point, Hernández Vásquez appears disoriented, placing his left hand on his stomach. 

After sitting for a moment, he can be seen lying down on his face on the concrete surface, with his legs moving. Hernández Vásquez then gets up and moves out of the camera's view for several minutes. At 1:24 a.m., he comes back into view, and the footage shows him collapsing with his face facing the floor. The boy no longer has his shoes on. 

At around 1:36 a.m., Hernández Vásquez gets back up one last time and walks towards the toilet before again seeming to collapse, this time on his back. The teenager can be seen moving his arms and legs, but by 1:39 a.m., his body was motionless. 

ProPublica said the footage, which Border Patrol handed over to the police, cuts off at around 1:47 a.m, with a four-hour gap before the footage starts again at around 5:48 a.m. Hernández Vásquez's motionless body is in the same position and place. At 6:05 a.m., the other migrant boy in the cell wakes up, notices Hernández Vásquez's body and goes to the cell door to get the attention of officers outside.

At around 6:08 a.m., a person ProPublica said is a Border Patrol physician assistant determines the teenager's body has no pulse.

According to Border Patrol information, an officer conducted three so-called "welfare checks" on Hernández Vásquez during the four-hour gap in footage. Customs and Border Protection (CBP), which oversees Border Patrol officers, has not explained the missing footage.

In addition to documenting Hernández Vásquez's last hours, the footage shows that his cellmate, the other migrant boy, was the one who first found his body. In May, CBP said he "was found un-responsive this morning during a welfare check."

Asked about the apparent discrepancy, a CBP spokesperson said the internal probe into Hernández Vásquez's death is ongoing. "While we cannot discuss specific information or details of this investigation, we can tell you that the Department of Homeland Security and this agency are looking into all aspects of this case to ensure all procedures were followed."

"Why did it happen, and why did his brother die?"

The video released on Thursday is likely to fuel even more questions and concerns about the measures Border Patrol took before Hernández Vásquez's death and during his last hours in visible agony. 

After leaving his hometown in central Guatemala, the teenager was apprehended at the U.S.-Mexico border near Hidalgo, Texas, on May 13. 

Although the agency is generally required by law to transfer unaccompanied migrant children to the Department of Health and Human Services (HHS) within 72 hours of their apprehension, Border Patrol kept Hernández Vásquez in its custody for a week, citing the surge of migrants at the time that overwhelmed its facilities for months. 

It was during this time the teenager fell ill and developed the symptoms of the H1N1 strain of influenza, also known as swine flu. 

H1N1 is particularly dangerous for young people, but is easily treatable if caught early, according to Purvi Parikh, a doctor and clinical assistant professor of pediatric infectious disease at New York University Langone Health. "We try to give an antiviral within the first 48 hours, because if you miss that window, it's much less effective," Parikh said. "The virus can move much faster and it can involve multiple organ systems, too."

Hernández Vásquez's autopsy showed that by the time of his death, he was also suffering from a series of bacterial infections, as well as pneumonia. Parikh said those infections are typically caused by an untreated flu that worsens over the course of at least a few days.

She noted that factors such as lack of sleep, cramped conditions and malnutrition could all hurt the body's ability to fight such an illness. "Sleep deprivation, fatigue, poor nutrition, all of that further depresses your immunity." Parikh said. "It makes you much more likely to get sick and then it makes it much harder for you to fight anything that you catch."

Efren Olivares, an attorney for Hernández Vásquez's family, told CBS News in an interview in June that Border Patrol showed a "disregard for the well-being of" children in its custody.

"I've talked to his brother and those are very difficult conversations. He has a lot of questions about what happened," Olives said. "Why did it happen, and why did his brother die? Why didn't they provide the proper care for him? And I don't have any of those answers."

Had Hernández Vásquez been transferred to the custody of the Office of Refugee Resettlement (ORR), the agency within HHS in charge of caring for unaccompanied migrant children, within 72 hours of his apprehension, he would have received a comprehensive medical exam within 48 hours. Border Patrol typically provides basic medical care, but children in ORR care are subject to far greater medical scrutiny, including influenza vaccinations not provided by CBP. 

Instead, less than a day before his death, a nurse found the boy had a 103 degree fever, according to his autopsy. He was prescribed the antiviral flu Tamiflu, but it was too late.

Olivares said in June he believed if ORR had received the boy on time, he'd still be alive.

"ORR specializes in handling children and taking care of children who have been traveling alone for days or weeks as they make their way to the U.S. That's their specialty. CBP does not have that specialization," Olivares said. "So I'm 100% convinced that had proper protocol been followed in this case, Carlos would be alive."