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

The pair were found in a hotel room in the community of El Pescadero on Mexico’s Baja California peninsula, according to the state attorney in Baja.

The man has not been publicly identified.

Initial forensic examination indicated that the couple died of intoxication by a substance, which authorities are still working to identify.

Lutz’s family said in their GoFundMe campaign for funeral costs that authorities informed them the couple may have died of carbon monoxide poisoning due to improper ventilation.

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

“Saturday she got really sick, and they thought it was food poisoning,” Slate said. “She wasn’t getting any better, so they took her to the ER, and she got some help there and went back to the hotel feeling good.”

Her family believed the couple were improving and enjoying the rest of their vacation until they got a call from the U.S. State Department, stepmother Racquel Lutz said.

“They just thought it was food poisoning,” she said. “They had no idea. None of us thought about that. You know, because you can’t smell carbon monoxide.”

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

“The safety and security of our guests and colleagues is always a top priority,” Gil said. “Local authorities confirmed there was no evidence of violence related to this isolated incident, and there is no threat to guests’ safety or wellbeing at this time.”

Gil, who said the hotel would refrain from speculation on the cause of death, referred further inquiries to local authorities.

Slate said that her stepsister was a frequent traveler and that the couple was very familiar with Mexico. The family knew that both Abby Lutz and her boyfriend were smart travelers and that they frequented only areas that they believed were safe.

“I think that’s the hardest part is going to a resort that you think you’re going to be taken care of at,” Slate said.

Racquel Lutz said, “It was just so senseless, and it could have been prevented.”

Carbon monoxide was responsible for two incidents last year in which Americans died of the noxious gas during vacations.

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

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

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

Her stepmother described her as an animal lover who was always excited to give out gifts on Christmas, making “knick-knacks” for everyone at her pottery class.

“I’d always wanted more children, and I got Abby when Tony and I got married,” she said. “And that was one of the most wonderful parts to have another daughter to be able to share life with.”

Slate said she’ll 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 anyone,” Slate said. “She’s the happiest, positive person. She loved everyone around her.”