A youth hacked a one-year-old boy to death and injured three others in Boktaboli area of the sadar upazila on Wednesday night.
The attacker is a paternal cousin of the victim, Azizul Haque, son of Imam Ali.
The injured are Tanzila, 10, and Tania, 9, two sisters of Azizul, and one of their neighbours, Razia Begum, 32.
The two sisters were admitted to Dhaka Medical College Hospital in a critical condition, while Razia was under treatment in Khanpur 200-bed hospital in the district town.
Police arrested the accused, Shamim, 26, early yesterday, and described him as a mentally challenged youth.
Abdul Matin, officer-in-charge of Fatullah Model Police Station, said that all on a sudden, Shamim entered the house of Imam and started stabbing Azizul with a sharp weapon.
As Tania and Tanzila, two sisters of the victim and their neighbour Razia came to resist Shamim, he stabbed them as well, leaving all of them critically injured, he said.
Shamim fled the scene soon afterwards.
Locals rushed the injured to Khanpur 200-bed hospital where doctors declared Azizul dead.
The victims' mother Sultana Begum said the incident occurred soon after she went out of doors. Sultana's husband Imam Ali is an expatriate in Singapore.