There is no clear motive why a man was stabbed to death by an intruder dressed in Santa gear, detectives said.
Police found the attacker's hat on Saturday morning
Sikander Shaheen, 25, was attacked in his bed by a knifeman wearing a red and white hat and full white beard.
Officers said they found no reason for the murder of Mr Shaheen, who was a private, hard-working, religious man, but the knifeman "intended to kill".
Neighbours called police to the house in Leyton, east London, on Saturday morning. Mr Shaheen died in hospital.
He was stabbed several times by the killer who had gained access to the shared house in Grove Green Road.
Police said there was no sign of forced entry or a struggle and nothing had been stolen from the property.
Det Ch Insp Keith Garnish said "I am convinced that person went in there with the sole purpose of killing someone, he had one thing on his mind."
DCI Garnish said he wanted to speak to Mr Shaheen's friends, colleagues and fellow worshippers to establish his lifestyle and a possible motive for the killing.
Police also wanted information about his movements after he came home from work on Friday evening.
The Santa hat thought to have been worn by the attacker was found nearby and is being forensically examined.