Despite spending the whole day in the Thompson residence, Brisbane police had not uncovered yet any motive or evidence to explain the murder of 14-year-old Sidonie Thompson, suspected to have been done by her own mother.

To better understand the circumstances, police were preparing on Thursday to review security footage captured by cameras on Story Bridge, where the suspect behind the murder jumped off, but left her 12-year-old son inside their hastily parked car. Sidonie's mother's body was found at the bottom of the river underneath the bridge.

The bridge had 34 cameras operated by the council; eight of the cameras pan and zoom at the span. The council had provided the police footage from the camera to help solve the mystery of the two deaths.

So far the only conclusion that they have reached is that the girl died from unnatural causes, but the authorities could not confirm if her death was caused by the multiple axe stabs that the Year 10 student from Somerville House sustained on Wednesday.

Autopsies which started Thursday on the corpses of the mother and daughter by forensic pathologists are expected to yield more information on the causes of death, but unlikely will provide any further lead on the motive behind the slaying.

One clue that could lead to explain the gruesome death is the woman's reported history of mental illness. The Australian, which identified the mother as 48-year-old Kim Patterson, reports that there was no major conflict between the mother and daughter, except for the usual tensions that arise between teens and their parents.

Another possible lead is the return of the Thompsons to Australia five years ago after Peter Thompson resigned from his post in London as head of Swiss bank UBS's European Corporate Finance unit.

While the police continue to search the Thompson house for more clues and evidence behind the two deaths that rocked Australia, floral tributes poured at the family residence on Carrington Street in upscale Paddington. The area, however, remains closed to non-residents who could not present proof of residence in Paddington.

Somerville House instead held a small chapel service on Thursday in which students left flowers and condolence cards and wrote on memorial books as tributes to their fallen but well-loved schoolmate.

"Somerville House is a close knit community and our students and staff have been deeply affected by the loss of Sidonie," Brisbane Times quoted Somerville Principal Florence Kearney's letter to Mr Thompson.

To further express their grief, students from Somerville will wear black arm bands, while the school flew the Australian flag on a half-mast. Counsellor and chaplain services were made available to students to help them deal with the tragedy. The school will hold a memorial service for Sidonie early next week.

There was also a gathering of family members at the house of George Patterson in south-east Queensland. Mr Patterson is grieving for the double loss of his daughter Kim and granddaughter Sidonie.

Ms Kearney disclosed that Sidonie, who wanted to become a neurosurgeon and was an excellent music student, also played badminton and sang in the school choir.

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