"As many as 25 police teams have been put in this case and the initial postmortem report obtained today suggests that all three victims were strangulated," DCP (Central) Paramaditya said.
A 50-year-old government employee-turned-property dealer Sanjay, his wife Jyoti (48) and son Pawan (21), an engineering student, were found murdered at their third floor residence in Old Rajinder Nagar yesterday.
"We have questioned as many as 40 persons today and zeroed in on five of them, presently considered the prime suspects. The suspects, including Sanjay's younger brother, are all close to the family," said an official privy to the investigation.
It is also suspected that at least one of the assailants had joined the family over dinner, as four plates with leftover food were found at the crime spot.
However, the victims were given no sedatives or drugs before the attack, the initial report suggests, the official added.
Police further said that they are carrying out technical surveillance and footage from all CCTV cameras near the building are being monitored, though the main camera, which could have directly helped the police in ascertaining the identity of the accused, was found to be dysfunctional.