Two men who tied up a young mother at her home and set it on fire would have been aware her baby was in the room, police have said.
Four-month-old Alexander Gallon died in the fire
Four-month-old Alexander Gallon died in the fire in Cowgate in Newcastle on Sunday night.
His mother, Danielle Wails, 21, was bound with a telephone cord but managed to dial 999 with her tongue and scream for help through the letterbox.
She was treated in hospital for minor injuries and has now been released.
Det Supt Barbara Franklin, of Northumbria Police, said: "The baby was crying, he was in the same room as the mother, and it is a tiny living room. They could not have not known the baby was in the house."
Police have launched a murder inquiry and are appealing for people with information to come forward.
Det Supt Barbara Franklin called the arsonists "wicked"
Det Supt Franklin said: "It is difficult to comprehend the wickedness of someone who could do such a thing and we appeal for the public to come forward and help us.
"This is a horrific incident which has left a baby dead and a young mum injured."
The mother and her baby had been at home alone on Sunday, with the front door left open, which was normal.
Ms Wails was in the kitchen at about 2230 BST when two men wearing black came in and punched and kicked her, Det Supt Franklin said.
It is believed she passed out after falling to the ground and was dragged to the living room where her son was sleeping on the sofa.
When Ms Wails regained consciousness the room was filling with smoke and she was tied up, police said.
She screamed and neighbours tried to get into the house but were unable to.
Ms Wails' hands were tied and she called emergency services using her tongue, police said.
Fire crews rescued Ms Wails and her son. They were taken to Newcastle General Hospital but Alexander was pronounced dead.
Ms Wails is currently staying with relatives. A post-mortem examination is due to be carried out on the baby on Tuesday.
The baby's father, Robert Gallon, 21, did not live with Ms Wails, but played an active part in his son's life. Police said he was distraught.
Police said they had not yet been able to establish a motive and there were several lines of inquiry.
Footage from a CCTV camera in the area is being examined.
The property is owned by a charity that houses single parents and pregnant woman, and Ms Wails had been living there for a few weeks.