Flavio Sandro Olmo, 42, murdered his wife and three children in the Brazilian municipality of Sao Domingos do Norte in the early hours of June 15.
The victims have been identified as wife, Eusivania Marcelino de Souza, 40, daughter, Laisla de Souza Olmo, 18, son, Italo de Souza Olmo, 8, and daughter, Anelise de Souza Olmo, 4.
Flavio allegedly waited for his family to fall asleep before killing them with a sledgehammer weighing 3kg (6.6 lbs) and a knife, police say.
Lieutenant Gabrine de Andrade told G1: “The suspect would have waited for the family to sleep and with a sledgehammer, he executed the family.
He then went to his room and hung himself.
The victims’ relatives claim the parents were in the process of separating.
The crime was discovered by Flavio’s brother, after his youngest niece, Anelise, didn’t pass his house on her way to school at around 7.30am as she usually would.
The uncle, who lives nearby, went to check on the family and was greeted by the grim scene.
He also told officers that his brother had taken medication for depression in the past. However, he had appeared to have overcome it before the crime took place.
Flavio had never been reported for violence, nor did he have a criminal record.
Civil police are believed to still be at the crime scene as officers continue investigating the case.
No further details have been released.