There were no signs of trauma, which means Gutierrez had almost certainly died of hyperthermia.
If I left them there too long, hyperthermia—heat stress—would set in.
Two of them had probably died like Diego, due to the hyperthermia.
The organizations annually update topics such as hyperthermia, hypothermia and hyponatremia.
hyperthermia hy·per·ther·mi·a (hī'pər-thûr'mē-ə)
Extremely high fever or body temperature, especially when therapeutically induced.
An abnormally high body temperature, usually resulting from infection, certain drugs and medications, or head injury. Hyperthermia is sometimes created intentionally to treat diseases, especially some cancers. Compare hypothermia.