A 15-year-old girl has died following an arson attack which left four family members critically ill.
Maleha Masud died on Thursday night, four days after the attack in Tooting, south London.
Her brothers Zain, 22, and Junaid, 16, and her sister Nabiha, 21, remain in a critical condition. Their mother Rubina, 53, has left hospital.
Police have started a murder inquiry. Two youths, aged 14 and 15, have been arrested and bailed.
Rubina Masud said: "I am devastated at what's happened. I can't believe anyone would do such a thing.
"I am just really concerned for my children. Please pray for them."
The London Fire Brigade was called to the property just after 0430 BST on Sunday and rescued Nabiha, Maleha and Junaid.
A mother has had her youngest child taken from her in the most tragic circumstances
Det Ch Insp Damian Allain
City banker Zain and Ms Masud escaped by jumping from a first-floor window of the house in Lessingham Avenue.
They all suffered burns and smoke inhalation.
Det Ch Insp Damian Allain said petrol had been poured through the letterbox before the fire had been started.
"A mother has had her youngest child taken from her in the most tragic circumstances," he added.
"Her three other children remain seriously ill in hospital. They were at the place where they should feel safest, in their family home.
"Whoever has committed this act is not only a coward but is a dangerous individual who must be caught."
He added: "Hundreds of cyclists would have been in the area at the time making their way to the start point of the London to Brighton bike ride at Clapham Common.
"I would appeal to people who may have seen anyone carrying a petrol can or similar container to come forward."
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