The teenage sister of a seven-year-old boy who died in an arson attack has been released from hospital, police say.

Joel Urhie was found dead after the blaze at his family home in Adolphus Street, Deptford, on Tuesday (August 7).

His mother, Sophie, and 19-year-old sister escaped by jumping out of a first-floor window and were taken to hospital with non-life threatening injuries.

Police said on Wednesday that Joel's sister has been released from hospital, where her mother is still being treated.

READ: Everything we know so far about the arson murder of little Joel Urhie in Deptford​

A post-mortem found the cause of Joel's death was "consistent with fire and smoke inhalation", officers said.

Detective Chief Inspector Mark Wrigley said police are keeping an "open mind concerning motive" to the murder and that speculation about the incident "is not helpful".

He appealed for anyone with information to come forward.

"This horrific incident has left a family shattered and the tragic loss of such a young life in suspicious circumstances resonates throughout the community in Deptford," he said.

"Our sympathies are with Joel's family and friends at this tragic time as they struggle to come to terms with their loss.

"I would directly appeal to anyone who was in the area and saw anything suspicious at around 3:30am, or who has any information about who was responsible for starting the fire that claimed to the life of an innocent child, to contact police immediately."

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Joel Urhie. Photo: Thomas Hornall/PA Wire

READ: Joel Urhie's dad describes heartbreak after Deptford blaze

CCTV footage which may hold vital clues to Joel's murder was seized by detectives on Wednesday morning.

CID officers removed the box connected to the camera system from a flat on the other side of the road from Joel's family home.

The camera does not point at the Urhie house but may have captured the person or people who started the blaze as they arrived at or left the scene.

Meanwhile a forensic team was combing the area for clues while uniformed officers conducted house-to-house enquiries.

Floral tributes have been left on the fence opposite the fire-ravaged house on Adolphus Street, where six fire engines and around 35 firefighters tackled the blaze after they were called at 3.25am on Tuesday.

Nearby properties were evacuated as a precaution and crews brought the fire under control just before 4.50am.