Kolkata: The couple murdered at Narendrapur in South 24-Parganas were strangulated to death.On Wednesday, the autopsy surgeon opined that the duo was hit on their heads with some heavy object and then strangled. During the investigation, police questioned the neighbours of the couple identified as Pradip Biswas (55) and Alpana Biswas (45) and the owners of the house and land where the couple used to live as caretakers. From sources police came to know that recently there were a plan to set up a factory in the vacant land. Also Read – Bengal family worships Muslim girl as Goddess Durga in Kumari PujaIt is suspected that there might be some conflict with the proposed factory owners and the deceased persons which resulted in killings. On Wednesday experts from Forensic Science Laboratory (FSL) visited the murder spot and collected samples such as pieces of cloth, a pillow, scissors and blood stains inside the house. It is suspected that 4-5 persons are connected with the murders. Also on Wednesday police found an envelope of a mobile phone SIM card. The police are trying to find out the owner of the SIM card. Also, the lost mobile phones of Pradip and Alpana are yet to be traced. Also Read – Bengal civic volunteer dies in road mishap on national highwayIt is suspected that the couple has been murdered by professional killers as the style of murder was gruesome. On Tuesday police, stated that only a TV was missing from the house which is suspected to be a trick by the murderers in order to make the situation complex for the investigators. On Tuesday, and bodies of Pradip and Alpana were found stuffed inside two suitcases in the washroom of the house where they were working as caretakers since 1997. One of the relatives of the couple went to the house as he was unable to contact the duo killed since Monday morning. It is suspected that Pradip and Alpana have been killed on Sunday night.