Ahdieh Yazdanparast, 46, died in hospital late last night from injuries she sustained in a fire at Venus Hair & Beauty in Stirling yesterday morning.
A man was also seriously injured in the fire, which started in the cellar of the Maxwell Street salon.
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The injured man was given first aid at the scene by two men until emergency services arrived, and both victims were taken to Forth Valley Royal Infirmary, in Larbert, for treatment.
Detective Superintendent Gary Flannigan, who is in charge of the investigation, said: "This is now a murder investigation and I would appeal to any witnesses who have not already spoken to the police to contact us as soon as possible by calling 101."
Police said they are not looking for anyone in connection with the fire, although patrols in Stirling were stepped up last night.
Police had issued an earlier appeal to speak to the men seen giving first aid, who were wearing a check shirt and a dark jacket.
A man with a brown top who passed with a bike, a lady who left the salon shortly before the fire, a man with a light jacket and a woman with long boots were also asked to come forward last night.
Chief Superintendent David Flynn, of Police Scotland's Forth Valley Division, said: "I would like to reassure the public that we are not looking for anyone in connection with this fire at this time."