A house fire in Derby which killed six children from the same family was started deliberately, police confirmed.
Five children aged between five and 10 died in the early hours of Friday after the blaze at a property on Victory Road in the Allenton area.
A sixth child, 13-year-old Duwayne Philpott, died in hospital overnight on Sunday.
Derbyshire Police said forensic examination of the house was ongoing but they were satisfied it was arson.
Assistant Chief Constable Steve Cotterill said: "Initial indications are that it was deliberately set and as a result six children have been unlawfully killed.
"The forensic examination is still continuing.
"A number of specialists, including fingerprints officers have been examining the scene and this is likely to continue for some time.
"We have yet to speak in detail with Mr and Mrs Philpott and that will happen sometime this week as understandably the couple are distraught at the loss of their six children."
Jade Philpott, 10, and her brothers John, nine, Jack, seven, Jessie, six, and Jayden, five, all died of smoke inhalation.
Duwayne had received treatment at Birmingham Children's hospital before his life support machine was switched off.
Their father Mick Philpott made "valiant attempts" to save them, according to the emergency services.
The family attracted attention in 2006 when Mr Philpott asked the council for a larger house to accommodate his wife, girlfriend and 14 children.
In 2007, ex-Conservative MP Ann Widdecombe spent a week living with the family as part of an ITV documentary and Mr Philpott also appeared on the Jeremy Kyle Show.
Police have renewed an appeal for information from anyone who was in or around Victory Road between 03:00 BST and 04:00 BST on Friday.
They also said any suspicious comments seen on social media websites should be reported.
A man, 38, and woman, 28, were released without charge on Saturday after being held on suspicion of murder.