The three expatriates, who were rescued in an unconscious state from a rented apartment in Sylhet, died due to lack of oxygen, says police.
On 26 July, five members of a UK expatriate family were rescued unconscious from their bedrooms at a house at Osmani Nagar in Sylhet.
Police rescued them breaking down the doors and then sent them to a hospital where on-duty doctors pronounced Rafiqul Islam and his son dead.
After 11 days, Rafiqul's daughter Sadia also died while undergoing treatment at a hospital.
Speaking with the press on Friday, Sylhet Superintendent of Police Mohammad Farid Uddin Ahmad, "We, after carefully examining the autopsy, viscera and other reports submitted on Thursday, have come to the conclusion that a shortage of oxygen was the cause of death of the three Sylhet expats.
"We didn't find any evidence suggesting that they were poisoned. The deceased also didn't have any external or internal injuries."
"We have been saying since the beginning that it [the room from where the expatriates were rescued] had all the doors and windows shut when we rescued the victims. The five-member family had their generator running inside the one-room apartment as there was no electricity on the night of the incident.
"The smoke and fumes from the generator caused the three to die as it created a shortage of oxygen. We are sure about the cause of death. The medical reports are consistent with our investigation and findings," the police super added.
Earlier on 23 August, police described the death of the three expatriates in Sylhet as "an accident".
At a press conference, the Sylhet police super had said, "No evidence of any kind of chemical reaction or presence of poison was found in the bodies of the three expatriates in viscera examinations."
Mohammad Farid Uddin had said that during the investigation police questioned the relatives and neighbours of the expatriates, but no proof of previous enmity or attack was evident.
The police super also said the deaths might have happened due to smoke from the generator.
However, Rafiqul's wife Husnara Begum and son Sadiqul Islam returned home after recovering from sickness.
Deceased Rafique's brother-in-law Delwar Hossain later filed an unnatural death case with Osmani Nagar Police Station in this regard.