Investigators also found fingerprints on a window of the house in the Derby suburb of Allenton, The Sun reported.
"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," Assistant Chief Constable Steve Cotterill.
Five of the siblings -- Jade Philpott, 10, John, 9, Jack, 7, Jessie, 6, and Jayden, 5 -- died in the fire, which broke out as they slept early Friday. Mick and Mairead Philpott were told Sunday that Duwayne, 13, could not be saved, and his life support was turned off.
Mick Philpott, who had a total of 17 children, was not at home when the fire broke out and had to be restrained from entering the house.
Philpott was involved in a custody dispute with Lisa Willis, mother of five of his children. A woman and man were detained for questioning and released without charge.