Ana Clara Benevides Machado passed out during Taylor's Rio de Janeiro concert Nov. 17 amid the city's oppressive heat -- 105 degrees -- and was pronounced dead hours later at the hospital.

Rio's Forensic Medical Institute now reveals heat exposure led to a "serious compromise of her lungs and sudden death" -- basically, in medical terms, she suffered a cardiorespiratory arrest.

The report notes she didn't have any preexisting conditions or substance abuse issues that could have contributed to her death.

Rio's public prosecutor has now opened a criminal investigation of her death ... saying in a statement Wednesday representatives of the concert's organizers, Time4Fun, will be called to testify following an in-depth analysis of the report.

Soon after the tragic incident unfolded at Estádio Olímpico Nilton Santos ... fans demanded the organizers supply more water at the following shows because they weren't allowed to take any into the venue themselves.

The company execs acknowledged their mistake shortly after ... saying they "did not economize in our efforts or resources to follow the best global practices in our industry to guarantee the comfort and safety of all."