A 21-year-old man has been arrested on suspicion of murder following the discovery of two bodies at a bungalow in Fulwood.
Two women were found dead in Longfield and the man, from Preston, was arrested at the scene.
Post mortem examinations will be carried out to establish the causes of death.
Police were called to a house on Longfield following a report of a fire at around 2am on Monday morning.
Lancashire Fire and Rescue Service and the ambulance service were also in attendance and the bodies of two women were found inside the property. The arrested man is the nephew of one of the victims who is in her 50s, and the grandson of the other, who is believed to be older.
Detective Superintendent Neil Esseen said: “We are still in the early stages of this enquiry and are trying to establish the circumstances surrounding the deaths of the two women.
“I have a team of detectives working on this investigation who are following a number of lines of enquiry.
“I would urge anybody who has any information about this incident to contact police as a matter of urgency.
“I am also keen to speak with anybody who lives in the Longfield area who may have seen or heard anything suspicious in the past 24 hours. Even the smallest amount of information, no matter how insignificant it may seem, could be crucial to this investigation.”