A house fire in which six siblings died as they slept in their beds is believed to have been deliberately started, police said.
Jade Philpott, 10, and brothers John, nine, Jack, seven, Jessie, six, Jayden, five, and 13-year-old Duwayne, all perished when the blaze ripped through their home in Allenton, Derby, in the early hours of Friday.
Assistant Chief Constable Steve Cotterill said: "After further forensic examination we believe the fire was not accidental, initial indications are that it was deliberately set and, as a result, six children have been unlawfully killed."
Results of post mortem examinations revealed the five younger children died as a result of smoke inhalation.
A cause of death has yet to be established in the case of Duwayne, who was taken straight to hospital following the fire but died overnight in the Birmingham Children's Hospital with his parents at his bedside.
Mr Cotterill said: "The forensic examination is still continuing.
"A number of specialists, including fingerprint officers, have been examining the scene and this is likely to continue for some time.
"We've 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."
A 28-year-old woman and 38-year-old man, both from Derby, were arrested by officers investigating the deaths but were released without charge. Police said they were still treating the fire as suspicious.
The children were asleep in their beds upstairs when the fire broke out at the semi-detached house in the early hours of the morning.
The children's father, Mr Philpott, made a "valiant" attempt to save them, police said.
Mr Philpott, said to be the father of 17 children, hit the headlines after demanding a larger house to share with his wife Mairead, girlfriend Lisa, and eight offspring.
He was dubbed "Shameless Mick" in 2007 for his benefit claims and refusal to get a job in a TV documentary by former Conservative MP, Ann Widdecombe.
Mr Cotterill thanked the members of the local community who had already approached police with information regarding the fire but renewed the appeal for information.
He added: "We would specifically like to speak to anyone who was in or around Victory Road between 3am and 4am on Friday morning."