Police's Criminal Investigation Department (CID) has claimed to have uncovered the mystery over the murder of four members of a family in Satkhira's Kalaroa upazila.
The CID said deceased Shahinur's younger brother Raihanul Islam alone carried out the grisly killings over family disputes.
Investigators also claimed to have recovered the machete Raihanul used to slaughter his brother, sister-in-law, nephew and niece. During integration, Raihanul said he killed them all as the devil told him to do so.
"We arrested Raihanul as a suspect and interrogated him after taking him on remand. He confessed to his crime," Additional Deputy Inspector General (DIG) of the CID (Khulna Range) Omar Farooq told a press conference in Satkhira Wednesday.
On 15 October, four members of the family were found dead with their throats slit. The dead were fish trader Shahinur Rahman, 40, his wife Sabina Khatun, 30, son Siam Hossain Mahi, 9, and daughter Tasnim, 6.
Shahinur and Sabina's five-month-old daughter Maria was also in the room but she survived the bloodshed.
DIG Omar Farooq said, "The murders took place mainly due to family disputes. Raihanul had no work and used to live with his brother's family. His wife also left him around a year ago."
"His brother and sister-in-law used to scold him for not having any income. On 14 October, they had a quarrel, following which Raihanul decided to kill his brother and sister-in-law," he added.
The CID said Raihanul bought sedatives and mixed those with soft drinks on the night of the incident. He served the drinks to the family.
"Around 3:00am, he entered his brother's room while they were asleep. Raihanul stabbed the brother, strangled his throat with a towel and cut off the veins to confirm his death.
"The two kids woke up in the meantime as Raihanul attacked his sister-in-law. He killed the mother and the children too so that the murders remain untraceable," said CID.
Quoting the suspect, the CID said, "During interrogation, Raihanul said he had no control over himself during the incident. He said he committed the murders as the devil told him to do so."
The weapon he used in the murders was recovered from a nearby pond. The towel was also recovered from Raihanul's room.
The CID on Tuesday arrested three individuals – Abdur Razzak, Abdul Malek, and Asadul Islam – on suspicion. Razzak and Malek were neighbours of slain Shahinur, while Asadul was the victim's employee.
The CID officials said there was no evidence to suggest the trio's involvement in the murders. "So far, it looks like a single-handed work. I think they [Razzak, Malek and Asadul] would not be required for further questioning," he added.