Firefighters and detectives attended the scene on Heywood Street, Great Harwood shortly after 4.30am on Sunday
An arson probe has begun after a lit firework was posted through the front door of a home.
Firefighters and detectives attended the scene on Heywood Street, Great Harwood shortly after 4.30am on Sunday.
They found a burning firework wedged in the letterbox when they arrived.
All four occupants of the house were asleep when the firework was dropped.
A mum and her partner, her 17-year-old daughter and her 18-year-old boyfriend all escaped unharmed.
DS Julie Cross said Accrington CID is treating the incident very seriously.
She said: "It is the threat of actions like this that is the worrying thing. It seems very strange that someone would do this at 4.30am. Whether they have done it with the intention to burn the house down or as some sort of prank I don't know. We are dealing with it and treating as a serious incident."
The incident happened at around 4.30am on Sunday, December 16 and caused minor fire damage to the door.