Sajjad Ahmad, brother-in-law of Rafiqul, said Rafiqul came to the country with his family on 12 July for the treatment of his son Sadiqul.
After staying in Dhaka for six days, Rafiqul rented the house in Tajpur of the upazila on 18 July.
Rafiqul with his family slept in one room after having dinner on Monday night and his father-in-law Anfar Ali, mother-in-law Badrunnesa, brother-in-law Delwar Hossain and his wife Shobha Begum and their daughter Sabila Begum, 8, slept in other rooms.
Additional Superintendent of Sylhet Police MD Lutfar Rahman said they rescued the five people in unconscious condition from a house on Mangalchandi road in Tajpur Bazar around 11.30am after receiving a call from the National Emergency Service 999.
They were rushed to SOMCH where the father and the son died around 2.00pm, said the official.
SOMCH director brigadier general Mahbubur Rahman Bhuiyan said that the bodies were kept at the hospital morgue.
Besides, a medical Board has been formed headed by professor Shishir Ranjan Chakraborty for the treatment of three others as they are in critical condition, he added.
Superintendent of district police Farid Uddin said they might have died due to food poisoning. “An investigation is on in this regard.”
A team of Police Bureau of Investigation (PBI) has also reached the spot to collect evidence.
Rafiqul's father-in-law Anfar Ali, mother-in-law Badrunnesa, brother-in-law Delwar Hossain and his wife Shobha Begum were taken to police custody for interrogation.