A man died while trying in vain to rescue his daughter when their house was deliberately set alight in Wakefield, police have said.
Andrew Broadhead, 42, and eight-year-old Kiera died in the blaze at their home in Ash Crescent, Stanley, on Wednesday.
Detectives said Mr Broadhead got out of the property but went back to "save Kiera who was trapped inside" the fire.
They believe an "accelerant was poured through the letterbox".
A woman aged 35 and a 13-year-old girl who were also in the house were treated in hospital before being discharged.
Police think the arsonist may have been injured because the fire took hold so quickly.
They appealed to people who had seen anyone with suspicious burns to contact the force.
Det Ch Insp Nicola Bryar, of West Yorkshire Police, said: "This is a terrible incident which has robbed a family of a husband and their youngest daughter.
"The seat of the fire appears to have been by the front door and we are investigating the possibility that an accelerant was poured through the letter box.
"We know that this fire took hold extremely quickly and it is possible that the person who set this fire was themselves burned in doing so.
"Have you seen anyone with burns or burn injuries in the last 24 hours in circumstances which you found suspicious?
"If so, then please contact us as a matter of urgency."
Firefighters were called to the scene at about 04:20 BST.
Ch Sup Mabs Hussain, of Wakefield District Police, said: "It cannot be overstated how devastating this fire has been for the local community in Stanley.
"Several of our officers working on the inquiry have also been significantly and understandably affected by this tragedy."
Rachel Mace, Kiera's teaching assistant at Stanley St Peter's Primary School, paid tribute to "a vibrant and confident little girl" who was "quite a delight to know".