Fire crews were called to Barn Mead in Harlow, Essex, at 1.45am.
Firefighters pulled six members of the same family from the blazing home but the mother and her three young children were believed to be dead at the scene.
Essex Police confirmed they were treating the blaze as suspicious.
In a statement they said: ''The cause of the fire is under investigation and detectives are looking at the circumstances leading up to the fire, and ask that anyone who was in the area of Barn Mead in the early hours and has any information contacts police at Harlow police station on 101.''
A joint police and fire service investigation will be carried out to establish the cause of the blaze.