Detectives from the Kent and Essex Serious Crime Directorate are treating the fatal fire at Barn Mead, Harlow as suspicious.
Police say the fire in Harlow in which a mother and four of her children were killed is suspicious.
Detectives are asking anyone who was in the area of Barn Mead or has any information to contact them.
Fire crews were called to the home around 1.40am.
Four children and their mother are now confirmed to have been killed.
Senior Investigating Officer Dectective Superintendent Rob Vinson said: "This is the still the early stages of our investigation into the tragic death of five members of one family, a mother and four of her young children.
"At this time we are investigating to see whether this fire was started deliberately and need anyone with information to contact us."
It has emerged that a car, not thought to belong to the family, was also alight nearby when crews arrived.
This fire is also being treated as arson.
Det Supt Vinson added: "For the sake of the family, who are trying to come to terms with this tragic incident, I would ask for information from the public, especially in the Harlow area, to help us determine exactly what has happened."