Michael Phillips, 68, and his wife Robbie, 65, who lived in Tennessee, were found dead on 7 May in their villa at the luxury Sandals Emerald Bay resort on the island of Exuma.
Vincent Chiarella, 64, was found dead in the adjoining villa; he was visiting the Bahamas with his wife, Donnis, who was discovered alive on a bed and rushed to hospital and later airlifted to Florida.
Autopsy and toxicology reports completed this month reveal that the three died of carbon monoxide poisoning, reported The Nassau Guardian.
The families of the victims have requested a private pathologist to conduct separate autopsies, health minister Michael Darville told reporters last week.
Forensic evidence and samples were collected from where the victims were discovered to determine whether any contaminants were present, according to the Bahamas commissioner of police Paul Rolle.
Officials said that the victims were staying in the same building, but different villas, in the same resort.
Officials added that there were no signs of trauma to their bodies.
Earlier Mr Rolle said that the Chiarella and Phillips couples both complained of feeling unwell on Thursday evening and received medical treatment before returning to their rooms.
“They were all treated at different times and they ate at different places,” he said.