DB said the motive was unearthed after interrogating five men who were directly involved in the incident.
The suspects are — Md Gulam Rabbi, Babul Prodhan alias Babu, Md Sohel Prodhan, Md Emon Hossain alias Hasan and Md Alamin Khandker alias Rihan.
Tobarak, 72, was killed at his flat in the city's Tejgaon Industrial Area last week. He lived with two personal assistants – Shahin and Shihab.
Briefing reporters at DMP media centre, Additional Commissioner (DB) Abdul Baten said DB (West Division) arrested the five men during raids across the country on Saturday.
During preliminary interrogation, DB learned that Tobarak had several shops at Mama Plaza of Mohakhali. Shahin and Shihab used to look after his business for several years. Shahin wanted to exact revenge on Tobarak after being sacked by his employer on specific charges.
Shahin got in touch with Sohel and told him that Tobarak carried money for business purposes and kept it at his flat. “So they decided to loot the money,” Additional Commissioner Baten said.
To execute the plan, they hired four people from Chandpur and another from Cumilla. They also arranged recruitment of Emon for taking care of Tobarak. Emon posed as Hasan, the son of Tobarak’s former caretaker late Mohammad Ali.
“The killers entered the flat with the help of Emon alias Hasan on early Wednesday and killed Tobarak when he was asleep. They fled the scene after looting the valuables,” the police officer said.