The daughter of a 75-year-old man who was stabbed to death on his way home from evening prayers said she believed it was a "premeditated attack".
Shazia Khan's father, Mohammed Saleem, was found after being stabbed three times in the back in Green Lane, Small Heath, Birmingham on Monday night.
Two sets of CCTV footage have been released from around the time the grandfather-of-22 was attacked.
West Midlands Police said they were puzzled by the motive of the attack.
'Unbearable and unbelievable'
The force said there was no evidence Mr Saleem had been the victim of a robbery or a racist attack.
Speaking at a press conference on Thursday, Mr Saleem's two daughters said it was "difficult to accept the brutality" of his death, which was "sad and devastating".
They said he was a "well-loved" and "respected" man within the community.
Mr Saleem's other daughter Nazia Maqsood said: "Nobody should have to go through this.
"The brutality that he had to suffer - he was an elderly man - is just unbearable, it's unbelievable."
CCTV footage shows a white male, aged between 25 and 32, seen running in nearby Wyndcliffe Road wearing a ribbed or striped "beanie-style hat" at about the time the stabbing occurred.
Police said that he could be central to their inquiries, but could also be unconnected.
Other footage revealed at the press conference shows a people carrier vehicle, near to the mosque on Green Lane, with two white men inside, both believed to be in their 30s.
The driver is believed to be of a "stocky build", and police believe both men could be significant witnesses.
Police are urging these people, or anyone who knows them, to come forward.
Det Supt Mark Payne said the police will do "everything they can to bring justice" following the killing, which happened in such a "brutal fashion."
He said: "Whoever is capable of doing this needs putting in jail and that's where we're going to put them."