A woman of 28 and a 38-year-old man, both from Derby, who were arrested in connection with the blaze, have been released without charge.
Smoke from the fire is believed to have rapidly funnelled upstairs, giving the children no chance to escape or for anyone to reach them alive.
Philpott, an unemployed father of 17, has regularly appeared in television programmes and newspapers as a supposed symbol of Britain's welfare benefits culture.
Detectives are investigating whether the attack was the result of a personal grudge against him and his family or linked to his wider "celebrity" in the media.
Derbyshire's Assistant Chief Constable, Steve Cotterill, said whoever lit the fire would have known children were most likely asleep inside.
"It is common knowledge in that community, if not nationally, that Mr and Mrs Philpott lived there with a large number of children," he said.
Police are urging anyone who finds a discarded petrol can, forecourt staff, or anyone who saw someone carrying fuel at the time of the tragedy to come forward.
Cotterill said the investigation was proceeding well but would take some time.
He said whoever was responsible for the fire should examine their conscience and hand themselves in.
Jade Philpott, 10, and brothers John, 9, Jack, 7, Jessie, 6, and Jayden, 5, all perished in the blaze. Duwayne Philpott, 13, died of his injuries in Birmingham Children's Hospital last weekend with his parents at his bedside.
Philpott said yesterday that the family had donated Duwayne's organs to help save the life of another child.
He also thanked members of the local community and people from across the world for their support since the tragedy.
"We grew up in a community that's been through a lot of problems with violence and to see this community come together like it has - it's too overwhelming."
- Independent
