Detectives have released pictures of a man seen fleeing the scene of the attack on 87-year-old Thomas O'Halloran armed with a knife.
The Met said that officers are keen to identify the man as a “matter of urgency”.
Mr O’Halloran was attacked in Cayton Road, Greenford near the A40 in west London at 4.06pm on Tuesday. No arrests have been made yet in connection with the killing.
In the picture, the man is seen wearing grey shorts, a dark T-shirt, a white baseball cap and white patterned builder-style gloves.
Detective Chief Inspector Jim Eastwood, leading the investigation, said: “We are keen to identify the man in the CCTV images as a matter of urgency.
”He is clearly a dangerous individual and people are advised not to approach him, but to call us immediately on 999 if they know him or have any information regarding his whereabouts.
”My team and I have been working around the clock to bring Mr O'Halloran's killer to justice, but we need your help. If you have any information at all that will assist us with our investigation - no matter how small - please contact us or alternatively, contact Crimestoppers anonymously.”
The Met’s appeal comes hours after O’Halloran was named as the victim of the stabbing - the capital’s sixth homicide in just four days.
Officers believe that Mr O’Halloran was stabbed in Western Avenue shortly after 4pm on Tuesday efore managing to travel 75 yards or so on his mobility scooter to Runnymede Gardens, where he flagged down a member of the public for help.
Locals told the Standard that Mr O’Halloran was a musician who would often play his accordion to raise money for Ukrainian refugees.
Hitesh Patel, 62, a shop owner at the nearby Greenford station, told the Standard: “He always had a Ukraine flag on the front of his basket. It said ‘Help for Ukraine’. He was a very good, kind man.
“I saw him just before he was killed heading up to the A40 on his scooter I was filling up with petrol but I wish I had said something. His wife was also always with him in her mobility scooter. He told me he was Irish and had three sons. It’s so sad. I can’t believe anyone would hurt him in broad daylight like that. We will miss him and his playing around here.”
Another local, Tony Paraskeba, 52, added: “It’s so sad. My son loved him. He would ask him to blow his horn on the scooter whenever he saw him and he would. He is so upset. Why would anyone hurt an old man?”
Chief Superintendent Sean Wilson, who is in charge of policing for west London, including Ealing, said: “Mr O’Halloran had his life needlessly snatched away in a shocking act of unprovoked violence.
“I understand that the community will, quite rightly, be shocked and appalled by this incident as I am too.
“Mr O’Halloran was a very well known and much loved member of our community. My thoughts are with his family and friends. I’d like to appeal for their privacy to be respected while they come to terms with their loss.”
