Police said a feud over distribution of booty led to the four murders in Keraniganj off the city. Dhaka district police super Habibur Rahman disclosed it to the press at his residence at 10:30 am on Friday.
Earlier, seven people were arrested in connection with the killings at the dead of night on Thursday.
The arrested are Rafiq, Jony, Sumon, Majid, Rani, Mukta and Shahida.
They were arrested from Srinagar in Munshiganj and Mirzapore in Gazipur.
The four—a couple and their two children—were found murdered at their home on Wednesday in South Keraniganj. Police suspected that the man was a member of a robber gang and that in-fighting had led to the killings.
The police super said the theft of a motorcycle of Sumon, distribution of booty and extramarital affairs led to the killings.