SYDNEY, Australia — David Dungay Jr., a prisoner at a Sydney jail, told the guards pinning him to his bed that he couldn’t breathe, according to a video recording. He said it at least 12 times, in a high, panicky scream.

One guard’s response to Mr. Dungay was also recorded: “If you’re talking, you can breathe.”

Minutes later, Mr. Dungay, a 26-year-old Indigenous Australian whose family’s lawyer says he had schizophrenia, diabetes and asthma, was unresponsive. He was declared dead about an hour after officers first entered his cell.

On Monday, the graphic video footage was shown on the first day of Mr. Dungay’s inquest, reigniting long-simmering anger about the deaths of Indigenous Australians in custody, which are a flash point of race relations in Australia.

Indigenous Australians are one of the most incarcerated groups on earth. Aboriginal and Torres Strait Islander people are 27 percent of Australia’s prison population, despite making up about 3 percent of the country’s total population.