A couple and their six-month-old daughter were found dead in their house at Balapara village in Gangachara Upazila on Saturday.
They were identified as Hafizul Islam, 30, son of Lal Miah, his wife Fatema Begum, 25, and their daughter Humayra.
The bodies were sent to Rangpur Medical College Hospital (RMCH) morgue for post-mortem examination.
Local union parishad chairman Afzalul Haque Raju said neighbours spotted the bodies and informed police.
Officer-in-charge of Gangachara Model Police Station Shushanto Kumar Sarker said they suspect Hafizul committed suicide after killing his wife and daughter over familial troubles.
A case was filed with Gangachara Model Police Station in this connection, OC said.
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