Jonathan Stock, 34, from Newcastle upon Tyne, England, stayed at home sleeping while his wife Meiling, 39, and their two children left for school in Battambang province on November 30 around 6:15 am.
However, when his wife returned home three hours later, Jonathan, wearing a blue shirt and black shorts, was found lifeless by the doorway of their home in the Sangke district of the city.
His shocked wife told police that a black plastic bag, found by investigators about 2ft away from the body, covered his head when she saw him.
She called her relatives for help and the bag was removed.
Deputy Provincial Police Commissioner Lim Puthyla, who is investigating the case, said: "Mr Stock was found dead by his wife at home. We are closely looking at the case.
"Our investigators will interview other people who knew the deceased. We will find the true cause of death.
"Currently we only have the testimony from one witness."
Ko Sun Kaing, the director of the Office of Serious Crimes, said police were also looking at suggestions Jonathan suffered a heart attack. However, they are waiting for the results of a post mortem examination.
He added: "I do not know the cause of death yet because I have not received the report from the district. I went down to the scene to observe the investigators working."
Speaking with police, Meiling said that she had left her husband alone in the bedroom.
She said: "I woke up my child and we went to her Chinese school. My husband asked to sleep more.
"After school I went to eat noodles at the market. When I arrived home around 9.20 am, I saw my husband dead by the doorway."
Meiling, who has been married to Christian missionary school teacher Jonathan for around 10 years, reportedly called her mother and brother for help and contacted the police.
Police and forensic experts went to perform an autopsy. The incident was first ruled out as suicide, but authorities were urged to probe more following speculations about his death.
Jonathan, who had lived in Cambodia for more than a decade, taught at the Christian SBS school in the province and authored several books on the Khmer language.
He was fluent in both speaking and writing the language and helped his wife run their coffee shop business.
Officers are now looking for more witnesses as they have only gathered information from wife, Meiling, who was the last to see Jonathan alive before she found him dead.
Meanwhile, the British Embassy in Phnom Penh said it had no objection to the family of Jonathan making funeral arrangements. They have also informed Jonathan's family in the UK.
It said: "We request relevant local authorities to assist in this matter and allow the family make proceed with funeral arrangement accordingly."