The convicts are—Altaf Hossain, his wife Farida, Jamir Ali, and Md Shahajahan Hossain.
Dhaka Seventh Additional Metropolitan Session’s Judge Md Nazrul Islam handed down the verdict.
According to the prosecution, the convicts abducted Kawser, son of Ibrahim, and demanded a ransom of Tk 50,000 over mobile phone in May 2012.
Although Ibrahim gave the ransom money to the criminals, they did not return his son back to him, it said.
Victim’s father filed an abduction case on May 26 later it was handed over to the Detective Branch of Police.
Some of the accused were arrested in the case but secured bail and demanded a ransom of Tk one lakh from the victim’s father again.
Later Jamir Ali was arrested on August 15 and three other suspects were also held. According to their statement, Kawser’s remains were recovered from a place near his house on August 18.