A domestic help working next door had allegedly masterminded the armed robbery in Salt Lake's CE Block last Friday in another chilling instance of a trusted hand turning traitor.

Sitaram Das, 30, had been to the Chakraborty family's house several times, sometimes carrying food from their neighbour's kitchen and to do odd jobs whenever they required him.

Nothing in his behaviour ever suggested he would turn on them. That illusion was shattered late on Tuesday with his arrest along with five of his alleged accomplices, one of them a co-worker. Three others are employed as domestic workers elsewhere in Sector I of Salt Lake. They were all traced to the homes they worked in, spread across neighbouring blocks.

One of the gang members is absconding, police said.

The arrested men are 22-year-old Rajesh Kumar Yadav, who has been working as a cook in the same house as Sitaram; Pappu Yadav, 22, Manoj Kumar Das, 18, Raja Halder, 34, and Tapan Purkait, 28.

Pappu is a familiar face for many in Salt Lake as a home-food delivery boy while Manoj is employed as a cook in a CF Block house. Raja and Tapan are van rickshaw-pullers. The duo were picked up from Raja's house in Jharkhali.

For the Chakrabortys, Sitaram emerging as the mastermind of the robbery has come as a bigger shock than the incident itself.

"I vividly remember the cold steel of one of the men's knives pressed against my throat. I was led from room to room and forced to hand over an envelope containing Rs 60,000 and jewellery. I can't believe it was Sitaram who plotted the entire thing," said Rinita Chakraborty, 23.

Her elder sister Namrata said it was hard to believe that someone they had known for four years was the leader of the pack. She described the incident as "the worst nightmare of my life".

Mallika, the siblings' mother, recounted that one of the masked robbers had appeared "familiar" since he was of the same height and build as Sitaram. But she didn't consider the possibility that it might be him.

This isn't an isolated incident of domestic workers turning to crime. Ranjit Chatterjee, an 82-year-old chartered accountant, was murdered in 2013 by Sona Das, a sweeper who had known him for 15 years. In 2010, Pratibha Dugar of Mandeville Gardens was robbed of her gold bangles and earrings by Prakash Yadav, her domestic help.

The Chakraborty family may be stunned by the revelation of Sitaram's criminal side, but police investigators were convinced from the start that the robbery was the handiwork of "an insider".

"We drew up a list of domestic workers in the area. We also started probing their acquaintances. When we started questioning them, Sitaram panicked and started calling his accomplices to alert them," said Santosh Pandey, deputy commissioner of the detective department in the Bidhannagar police commissionerate.

By then, the police were tracking Sitaram's mobile calls. A police officer said Sitaram had called Raja and Tapan, who stays in Kestopur, to advise them to flee with some of the loot. Two mobile phones and some of the stolen ornaments were found in Sitaram's room in the house next door, the officer confirmed.

The six arrested men were produced in a Salt Lake court on Wednesday. Sitaram and Rajesh were remanded in judicial custody and the rest in police custody for 14 days.