On the day they were murdered, Nusrat and Alvi were asked by their mother to go to bed, RAB media wing director Commander Mufti Mahmud Khan told a press briefing Thursday.
As Nusrat lay in bed, Mahfuza attempted to strangle her. Nusrat resisted but was finally overpowered. Mahfuza then strangled Alvi.
To cover up the crime, Mahfuza called her husband Amanullah, who was in Uttara, and her sister Afroza, who lived in a rented flat near Mahfuza's Banasree residence.
Mahfuza Malek Jesmin, 35, on Thursday confessed to strangling her two children to death.
Her motive remains unclear.
She has told investigators that a bout of depression brought on by anxiety about the siblings' future had caused her to murder them, Commander Mufti said.
Father of the victims Amanullah filed a murder case against Mahfuza around 9:45pm, OC Rafiqul Islam confirmed to the Dhaka Tribune.
Earlier, RAB informed the Dhaka Tribune: “Mahfuza killed her children over an extramarital affair and psychological and social disorder.”
The elite force now says it has not found evidence of an affair and that the double murder was the work of the psychologically afflicted mother.
But at the same time, investigators described Mahfuza as calm and coherent during questioning and while thoroughly describing the killings.
Investigators have not found any evidence that anybody else was present when the children were fatally assaulted, but Commander Mufti said a number of facts indicate that the family was involved in the murders or in the subsequent effort to cover it up.
Fourteen-year-old Nusrat Jahan and six-year-old Alvi Aman were declared dead at Dhaka Medical College Hospital (DMCH) after they had been brought there unconscious on Monday night.
At the time, family members claimed that the children had fallen asleep after having leftover Chinese food for lunch and did not wake up.
Conflicting accounts of the incident given by the parents and family members led forensics experts on Wednesday to suspect that the two siblings from Dhaka’s Banasree neighbourhood might have been murdered.
Mahfuza's decision to stay at home while her children were taken to a nearby hospital was described by investigators as “unusual and suspicious.”
The result of post-mortem examinations at Dhaka Medical College hinted at foul play.
Injury marks were found on the children’s necks. The boy also had injury marks on his legs and his sister had eye injuries.
Forensic scientists say they were waiting for the visceral test results to determine the causes of death.
RAB took eight people, including the victims' parents and maternal aunt Afroza Akhter, into custody in connection with the deaths of the two siblings. Five have since been released.
The children's parents and aunt remain in custody and are expected to be handed over to Rampura police.
Mother Mahfuza Malek Jesmin, father Amanullah and maternal aunt Afroza Akhter are expected to be handed over to Rampura police, Sub-Inspector Shomen Kumar Poddar, the investigating office for the case's First Information Report at Rampura police station, told the Dhaka Tribune last night.
A case would shortly be filed, he added.