A 27-year-old woman has been arrested on suspicion of child neglect after four children - two sets of twin boys aged three and four - died in a house fire in south London.
The youngsters were all related, police said.
The woman was arrested and remains in custody, a spokesman for the Metropolitan police said on Friday afternoon.
He confirmed that "there was nobody else inside the property at the time of the fire."
When firefighters arrived at the property in Sutton just before 7pm on Thursday the blaze was already "very well-developed", London Fire Brigade's (LFB) deputy commissioner, Richard Mills, told reporters.
Home Secretary Priti Patel said: "My thoughts and prayers are with the families and loved ones of the four young children who tragically lost their lives in the fire in Sutton yesterday. I am being kept updated by the emergency services and I would like to thank them for their work in such sad circumstances."
He said at the scene: "I can confirm that they are two sets of twins, ages four and ages three," adding that they were all boys.
They were carried out of the house and given CPR by firefighters before being taken to two south London hospitals where they died.
A message with flowers laid at the scene reads: "To Kyson Bryson Leyton + Logan. Sleep well. Love Grandad."
In a message on Facebook, Jason Hoath said the boys were "my amazing smart and beautiful grandsons" and he is "devastated".
They will "always be in our hearts and thoughts" and "boys I love you forever", he added. "Grandad xxx."
Deputy Commissioner Mills said: "On arrival there were four children in the house. It's a very traumatic event. They (firefighters) attended a very well-developed fire (and) got there in under three minutes. Second two machines within seven minutes.
"Entering a building and rescuing life is always a very difficult place to be. The age of the children (and) the fact they are so young will have a profound impact on them."
The cause of the fire is currently "unknown" and fire investigation teams, alongside the Metropolitan Police, are trying to determine the cause, Mr Mills said.
LFB said eight fire engines and around 60 firefighters were sent to Collingwood Road.
The fire was under control by 8.36pm.
A woman who works in a local shop said the children were "so well behaved" and "polite".
"They were always all holding hands," she added.
Westbourne Primary School in Sutton said it was "devastated" by the boy's deaths, two of whom were pupils there.
It said: "Westbourne Primary School and its entire community are devastated at the news of the tragic loss of the four children; our hearts, thoughts and prayers are with the family and anyone else affected by this heart-breaking event.
"We will continue to support the children, the staff, the families and the community through these very difficult times."
London Fire Commissioner Andy Roe said the deaths had "left everyone numb with profound sadness".
Mr Roe added: "My thoughts are with the family and friends of the children, the whole local community and all those who will be affected by this fire.
"Fire crews arrived quickly and located the children inside the property. They were brought out of the house and given immediate emergency care.
"The welfare of our staff involved in this incident is very important to us and all of them will be offered counselling as a priority."
'Devastating and tragic loss'
Superintendent Rob Shepherd said: "The thoughts of everyone at the South Area Command Unit are with the family and friends of these four young children, who have suffered such a devastating and tragic loss tonight.
"I know these sentiments will be shared by the local community, Sutton and London as a whole.
"We remain on scene with our emergency service colleagues and will be working through the night and beyond. At this very early stage the cause of the fire is unknown and will be investigated."