A man has died in hospital five days after a firework was set off in his house.
Anthony Nicholls suffered severe burns and smoke inhalation in the fire on Thursday and had been in an induced coma since.
The 56-year-old did not regain consciousness and died in the early hours of Tuesday morning, his family said.
Police believe the fire in the house in Birmingham was started by a firework and had previously said they were treating it as an arson attack.
Following Mr Nicholls' death, West Midlands Police have said they have now launched a murder inquiry.
Mr Nicholls' partner - a 50-year-old woman - remains in hospital following the fire after jumping from a first floor window in an attempt to escape the blaze and suffering fractures to her legs in the process.
Police believe the fire started after a large commercial-sized firework was placed in the hallway of the property and set alight.
It is thought to have contained around 200 tubes of explosives and would have taken around two minutes to fully discharge.
Police said the property in Birchtrees Drive in the Tile Cross area of the city, caught alight quickly and was gutted by the blaze.
Detective Inspector Paul Joyce, from the force's homicide team, said: "Sadly one of the victims of this fire has died and our thoughts are with his family and partner at their time of loss.
"His death means that we are now treating this as a murder enquiry and I would like to re-emphasise the need for anyone with information to call us.
"We are following up several active lines of enquiry and are focusing on CCTV from the local community. If you have any home CCTV that we have not yet seen please call us.
"The impact would have been terrifying for the occupants and I would urge anyone who sold or is missing a firework of this size to get in touch.
"This would have been a large heavy firework that would have been difficult to carry some distance, so I would ask anyone who saw someone carrying a large box in the area last Thursday night to contact us.
"Any information no matter how insignificant if may seem to you could help our investigation."
Anyone with information on the fire should contact West Midlands Police on 101 or Crimestoppers anonymously on 0800 555 111.