day after h learninger sister and her fiancee were found dead in their room at a resort in the mo gany man republic. today investirs announced their cause of death was respiratory failure.he in words, excess fluids on the >> the most likely indicates that there was some type of lung injury that led to basically leaking of fluid into the portions of the lung that should be filled withair. when that happens, people don't get enough oxygen and they can die. >> reporter: investigators are looking at the possibility that carbon monoxide poisoning can be blamed. >> it basically poisons the cells, so even though there's en enough oxy around, it's not being used properly and the cells lose their energy pump, so it's like a car without a motor. >> both families are having a hard time believing respiratory issues caused cynthia and nathaniel's death. they point to t photos likes that the couple was happy and healthy on vacation. just as she was friday, holmes daughter is struggling to make sense of all. >> it's still hard to believe. >> police in t dominican republic would not confirm whether th