A fourth child has died following a house fire which police are treating as suspicious.
Police and firefighters were called to the house in Barn Mead, Harlow, Essex, at about 1am this morning.
A woman and three of her children were taken to hospital, where they were pronounced dead.
A fourth child died later at the Princess Alexandra Hospital and a fifth remains in a critical condition.
A man has been treated for minor injuries.
Neighbours raised the alarm after being woken in the early hours. They reported seeing flames and smoke billowing from the house.
– Senior investigating officer Detective Superintendent Rob Vinson
This is the still the early stages of our investigation into the tragic deaths 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.
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.
The force confirmed that a car was also found alight a short distance from the house.
The vehicle was not connected to the family, who are believed to be of Asian origin; however, the car fire is also being treated as arson.
The house will be forensically examined to find out how the blaze started.
Firefighters called to the scene described the house as being "100% well alight", with flames coming from the front and back doors.
– Essex Fire and Rescue Service assistant divisional officer Steve Foster, incident commander
Crews arriving at the scene did everything they could. They were faced with a rapidly developed fire and went straight into the burning building wearing breathing apparatus to bring people out as quickly as possible.
Once casualties were out, they worked alongside ambulance teams to do all they could.
Fire investigation teams are now working with police to establish how this tragedy happened and our thoughts are with those the family left behind.
A spokeswoman for the service said the father was not at home when the blaze broke out.
The children who died at the scene are believed to have been two boys, aged six and 13, and an 11-year-old girl.
Sharon Pavey has lived in the house next door for 10 years. She said the family had only recently moved to the area and she believed the father worked at a hospital.
– Neighbour Sharon Pavey
I saw flames coming out of the back window and dialled 999 but they were already on their way.
The flames and smoke were billowing out and we could tell it was very serious.
I don't think many people knew them but we are all in shock and this is very sad news.