The family of Abby Lutz, one of two Americans found dead in Mexico on Tuesday, told NBC News on Thursday that they believe she and her boyfriend died of carbon monoxide poisoning while on vacation.

Abby Lutz was found with her boyfriend, John Heathco, in a hotel room in the community of El Pescadero in Mexico Baja California Peninsula, according to the state attorney in Baja.

Initial forensic examination indicated that the couple died of substance intoxication, although authorities are still working to determine what substance it was.

But Abby Lutz’s family said on their GoFundMe for funeral costs that they were told by authorities the couple may have died of carbon monoxide poisoning due to inadequate ventilation.

Abby Lutz.
Abby Lutz.Courtesy of the Lutz family

Abby Lutz’s stepsister, Gabrielle Slate, said the couple felt terribly ill over the weekend and went to the hospital where they were given IV fluids.

«He got really sick on Saturday and they thought it was food poisoning,» Slate said. «She wasn’t getting better, so she was taken to the emergency room and she got help there and she came back to the hotel feeling fine.»

Her family believed the couple was on the mend and enjoying the rest of their vacation until they got a call from the US State Department, stepmother Racquel Lutz said.

«They just thought it was food poisoning,» he said. «They had no idea. None of us thought of that. You know, because you can’t smell carbon monoxide.»

Henar Gil, general manager of Rancho Pescadero, offered his condolences to the family in a statement Thursday.

«The safety of our guests and colleagues is always a top priority,» Gil said. «Local authorities confirmed that there was no evidence of violence related to this isolated incident, and there is no threat to the safety or well-being of guests at this time.»

Gil also said the hotel would refrain from speculating on the cause of death and referred inquiries to local authorities.

Slate said her half-sister was a frequent traveler and the couple was very familiar with Mexico. The family knew that both Abby Lutz and her boyfriend were savvy travelers and only frequented areas they considered safe.

«I think the hardest part is going to a resort where you think you’re going to be taken care of,» Slate said.

«It was so absurd and it could have been prevented,» said Racquel Lutz.

Carbon monoxide was responsible for two incidents last year in which Americans died from the noxious gas while on vacation.

In May of last year, three tourists — Michael Phillips, 68; Robbie Phillips, 65; and Vincent Chiarella, 64, died at a Sandals resort in the Bahamas from carbon monoxide poisoning.

Months later, in November, a group of Americans staying at an Airbnb in Mexico City died the same way. Their families told NBC News at the time that they were surprised the rental company didn’t require hosts to have working carbon monoxide detectors.

Abby Lutz’s father, Tony Lutz, is devastated by her death, the family said Thursday. She had just made travel plans to visit him for Father’s Day next weekend.

Her stepmother described her as an animal lover who was always excited to give presents at Christmas, making «trinkets» for everyone in her pottery class.

«I always wanted more kids and I had Abby when Tony and I got married,» she said. «And that was one of the most wonderful parts of having another daughter to share life with.»

Slate said she will remember her sister for her sweetness and her love for her nieces and nephews.

«I don’t think I’ve ever heard her say a bad word about anybody,» Slate said. «She is the happiest and most positive person. She loved everyone around her.»