The incident took place yesterday and a person who worked as a driver with the agency was allegedly involved in the crime, they said.
The victims were carrying 3.5 kgs of gold and 0.86 kg of diamonds, the exact value of which was yet to be ascertained but would run into crores of rupees, Deputy Police Commissioner Pravinkumar Patil, said.
According to the DCP, the complainant Ramesh Kumar Gupta, security supervisor at S R Securities, and three others arrived at Borivali railway station by train from Surat, carrying the consignment of gold and diamonds belonging to a private firm. It was supposed to be delivered to the firm's branch in Bandra here.
At this point, at about 3.45 pm, four unidentified men who were carrying guns and some sharp weapons got into the vehicle and threatened Gupta and his colleagues. Singh was found to be in cahoots with them, the DCP added.
Singh then drove them to Virar in neighbouring Thane district where he and the four unidentified men abandoned the vehicle and fled with the consignment.