Three children are among five members of the same family to have died in a house fire in west London.
One person remains unaccounted for and one man is in hospital after the blaze in Channel Close, Hounslow, on Sunday.
The London Fire Brigade and the Met Police said they were keeping an "open mind" over the cause.
Ch Supt Sean Wilson said it was "truly a terrible incident" and the loss of lives was causing "unimaginable stress and pain".
The bodies of the five family members were found on the first floor of a terraced property by London Fire Brigade (LFB) crews.
The adults who lived at the home have been named locally as Aroen Kishen and his wife Seema.
"This is truly a terrible incident the loss of so many lives will cause unimaginable stress and pain to all the families involved," Ch Supt Wilson said.
"Everyone will want to know why this has happened and we will be working tirelessly with London Fire Brigade to find those answers."
He added that his officers would be there to support those in the community enduring "overwhelming sadness and shock of the tragic loss".
The Met also added that no arrests have been made.
Deputy fire commissioner Jonathan Smith said it was too early to know whether fireworks or candles for recent Diwali celebrations were a factor.
He said firefighters faced a "significant blaze" on the ground and first floor when they arrived.
Labour MP for Feltham and Heston, Seema Malhotra said she would be visiting the street later on Monday.
She said: "It is utterly devastating. I know that so many have been in touch with me are just… shocked and hugely saddened by this tragic incident in Heston last night.
"At the moment I have heard nothing about what might have caused this.
"There hasn't been any suggestion at the moment of what caused the scale of this fire and for it to go through the property so quickly."
She urged everyone to "keep an open mind" to what the cause of the fire was, as investigations were under way.
Residents in the area said they saw a lot of smoke at the time, but many fireworks were also being let off for Diwali.
Nick Marbrow, who lives on Sutton Road, said: "When I went to bed last night, I could see a lot of smoke.
"I could see an unusual amount of smoke, but then it is Diwali, there were fireworks going off."
Ashish Sosniah, who was passing the street on his way to temple, said he saw fireworks going off "for an hour" between 20:00 GMT and 21:00.
Mr Sosniah said that when he next passed at around 23:30 there were emergency services at the scene.
The mayor of London, Sadiq Khan, said: "My thoughts are with the victims, their loved ones and the wider community following this terrible fire in Hounslow in which five people tragically lost their lives including three children."
London Fire commissioner Andy Roe said: "This is a terribly sad incident and the thoughts of all of us at London Fire Brigade are with the family, friends and all those affected at this difficult time.
"Staff will be in the local community today to offer support and advice where needed.
"The welfare of our staff is very important and all those involved will be offered support from our counselling and trauma service."
Emergency services were called at 22:30 and the fire was under control by 01:25.
A total of 10 fire engines and around 70 firefighters were called to the blaze.
Nearby properties were evacuated as a precaution.