Officers revealed on Wednesday that the fire was started inside a wardrobe in one of the bedrooms at the house on Lytham Road, Freckleton.
Preliminary investigations by fire safety officers have revealed none of the house’s four smoke alarms were working at the time of the blaze.
Four people, Reece Smith, 19, four-year-old twin girls Holly and Ella Smith and Jordan Smith, two, were killed in the blaze. Post-mortem tests confirmed the four victims, found in two attic bedrooms at the house, all died from the effects of smoke inhalation.
Their mother, Michelle Smith, 36, was rescued from the property by firefighters and was not seriously injured. Mrs Smith, a mother of nine, had been celebrating her birthday on the night of the fire and had held a party in the house.
Detective Chief Inspector Neil Esseen, from Lancashire Police, said: “I can confirm that this fire is now being treated as suspicious.
“We are following a number of lines of inquiry and have spoken with a number of people who were at the house on the night in question. However, I would still urge anybody who has any information which they believe could assist us with our investigation to come forward.”