Sagar Sarwar and his wife Meherun Runi, news editor of private Maasranga Television and senior reporter of private ATN Bangla respectively, were chopped to death at their West Rajabazar residence in Dhaka early on Saturday morning, police said.
The bodies of the couple -- both in their late 30s were found by their five-year-old son, who called relatives living nearby, they said.
Police said the child phoned his maternal grandmother early in the morning and said "my parents are dead".
The motive for the killings was not known and it was unclear if the deaths were related to their work.
Doctors, who carried out the autopsy, said the killers stabbed Sarwar at least 20 times at different parts of his body and then confirmed his death by cutting his throat while Runi was stabbed twice.
"We think it (murder) was not an act of any professional killer ... no professional killer murdered the couple," a doctor familiar with the autopsy said.
Police said they initially detained four people for questioning as four teams already launched probe into the murders while Bangladesh Home Minster Sahara Khatun said law enforcement agencies were ordered to provide the report within 48 hours.
They said they were looking for clues as no valuables were missing from the house, meaning it was not a case of robbery.
Joint commissioner of detective branch of police Maruf Hassan said one of the four detainee was a brother-in-law of Sarwar while police earlier said the three others were manager and the two security guards of the apartment complex.
Runi's brother was later released after questioning, police sources said.
"We are laying highest priority to track down the perpetrators of this heinous crime," police commissioner of Dhaka Benajir Ahmed told reporters.
Different journalists organisations protested and demanded exemplary punishment of the assailants after a quick investigation as government and opposition leaders mourned the deaths.
The couple was buried at the city's Azimpur graveyard after their funeral prayers which was joined by hundreds of mourners including journalists and politicians.
Photograph: File image of Sagar Sarwar with his wife Meherun Runi