The autopsies of the two siblings, found dead at their flat in the capital's Banasree on Monday, suggest that "they might have been murdered".
"There were bruises on different parts of their bodies," said Assistant Prof Sohel Mahmud of forensic medicine department of Dhaka Medical College (DMC).
"Scratch marks, bruises and finger marks were found in the throats of both the kids.
"Their tongues were bitten," he told journalists after the autopsies yesterday, without specifying how the children could have been killed.
A DMC morgue source, however, said these signs are found only in case of strangulation.
Some of the marks on the children's bodies were caused by a scuffle before their death, the source added.
Nusrat Aman Aroni, 14, and her six-year-old borther Alvi Aman were found unconscious at their apartment on the fourth floor of a seven-storey building around 5:00pm, according to Mohammad Amanullah, father of the victims.
They were initially believed to have died from food poisoning.
Asked about that, Sohel Mahmud said they had collected viscera samples of the bodies and would send those for chemical tests.
"It may take one to one and a half months to get the reports ... Once we have those, we can confirm if the kids were poisoned."
Nusrat was a fifth grader at Viqarunnisa Noon School and Alvi a nursery student at Holly Crescent School in Banasree.
Amanullah and his wife Mahfuza Malek Jasmine went to their home in Jamalpur Sadar upazila yesterday morning.
Describing the day's incident, Amanullah, a garment accessory trader, told our Jamalpur correspondent that he was at his Uttar Badda office at the time of the incident.
"A male and a female teachers were supposed to come to tutor Nusrat -- one at 3:00pm and the other at 4:00pm. But on Monday, they incidentally came at 3:00pm together. My wife fell asleep after they arrived.
"When she woke up around 5:00pm, she found the teachers had left, the main door of the apartment had been left open and the children were lying on the floor unconscious," Amanullah continued.
"My wife first called her sister Afroza Malek Mila, who lives nearby, and she rushed to my house immediately. She then called me around 6:00pm and I instantly took a rickshaw home."
"On the way I called my friend Jahid, who lives in the same area, and asked him to go over to my house to check on my kids."
Jahid and Mila then rushed the children to Al Razi Islamia Hospital in Banasree around 6:30pm, he said.
As the hospital refused to admit the children, they were rushed to the DMCH where they were declared dead at 7:40pm.
Asked if he suspected anyone for the murders, Amanullah said, "As far as I recall, I have no such enemies."
Jasmine, the mother of the children, was too distressed to talk to the media.
Amanullah and Jasmine, who were cousins before they fell in love and eventually tied the knot, had been living in the apartment with their children since August last year, said building owner Abdul Quadir.
Amena Begum, a domestic help who has been working for the family two hours every day for the last five months, said she never saw the couple quarrelling.
After autopsies, the bodies were taken to their Jamalpur home and buried there last night.
Though the father had claimed that two tutors had gone to the house, Pintu Mondol, a security guard at the building, told The Daily Star he saw only one entering the flat.
"It was the female tutor. She entered the building just before 3:00pm and left around 4:15pm. This is her regular schedule. I found nothing suspicious in her gestures."
He said no one else had come to the apartment before he went to say his Maghrib prayers around 6:10pm.
"After I returned, two men came in a car and rushed to the apartment. They along with the children's aunt came downstairs hurriedly carrying the kids in their arms and drove off.
"I asked them what was wrong. They said the children had fallen ill and they were taking them to a hospital," he added.
This correspondent contacted the house tutor who, according to the guard, was the last person to leave the apartment before the children were found unconscious.
She said when she left the apartment around 4:15pm, Jasmine was sleeping in a room and Alvi was playing inside the apartment.
"And Nusrat opened the door for me and bid me goodbye," she said.
The identity of the tutor, aged around 25, is being withheld.
Asked why the Al Razi hospital refused to treat the children, a source there told this correspondent, "The moment we looked at the kids, we knew they were dead."
The family was yet to file any case but investigations were on, Officer-in-Charge Rafiqul Islam of Rampura Police Station said last night.
"We want to wait for the family to get over the loss and file a case. If they don't, we will lodge one."
No arrests were made either, he added.
The Rapid Action Battalion, meanwhile, picked up five people for quizzing.
They are the female tutor, family relatives Obaidul Islam and Shahin, and security guards Pintu and Ferdous, according to a text message sent to this newspaper by the Rab.