Police and neighbours at the Kings Road flat of the slain Varma couple on Sunday. A Telegraph picture

The probe into the murder of Howrah couple Ramprasad Varma and Sunita Devi centred around a pair of sandals on Thursday.

Neither of the couple’s sons — Sandip and Abhishek — could identify the sandals that police recovered from the Varmas’ Kings Road flat.

Suspecting that the sandals belonged to one of the killers, the sleuths brought a sniffer dog to the apartment around 9am on Thursday.

An officer entered the flat with the dog through a window as the door has been locked since the twin murder on Saturday night and the key hasn’t yet been traced.

After sniffing the sandals, the dog came out of the flat through the same window, climbed down the stairs to the ground floor and went to the rear of the apartment block before ending the trail at the parking lot near the entrance to the compound.

“Judging by the movement of the dog, it seems the killers had fled by scaling the boundary wall behind the apartment block,” said an officer of Golabari police station.

The sleuths are scanning the call lists of the couple’s cellphones, as well as those of the two sons and other family members. “Sunita Devi received the last call around 8pm on Saturday from Abhishek,” said an investigating officer.

Throughout the day, the sleuths interrogated the sons, their sister Sweta and her husband and Trinath Dalai, who had taken Ramprasad’s shop in the AC Market on Dobson Road on lease.

The two guards of the complex, hired a month ago, were also grilled. “We are sure that at least one of the killers was known to the couple,” the officer added.

“There is no breakthrough yet. We are examining all evidences,” said Milan Kanta Das, the additional superintendent of police.