The property was occupied by a 56-year-old woman, her daughters – 21 and 25 – and her granddaughter, aged three. They were awoken by the smoke alarm and made their way to a first floor window. One of the women jumped from a ledge under the window, went and got a ladder from a neighbour’s house and helped the others to escape to safety. No one was injured.
Detective Constable Neil Third, of Bournemouth CID, said: “This is an extremely serious incident and it was very fortunate that no one was hurt.
“Had the smoke alarm not been working we could be dealing with the deaths of four innocent people.
“I am keen to hear from anyone who saw anything suspicious in the area or knows of someone unusually smelling of petrol or smoke since this crime occurred.
“I urge anyone who has information about this incident or the offenders to contact Dorset Police urgently.