Officers investigating the blaze said they were not ruling any possibility out and are continuing to investigate the possibility that it was an arson attack.
But early results suggest the blaze started in the lounge at the rear of the house and that no accelerant, such as petrol, was used.
Doctor Sabah Usmani, her sons Sohaib, 11, and Rayyan, six, and 12-year-old daughter Hira died in their end-of-terrace house in Barn Mead, Harlow.
A third son, Muneeb, nine, and second daughter Maheen, three, were rescued by fire crews and taken to hospital in a critical condition but Muneeb later died.
Ms Usmani's husband Abdul Shakoor, also a doctor, suffered minor injuries as he fought to rescue his family from the blaze.
Detective Superintendent Rob Vinson said officers were continuing to investigate "many lines of inquiry."