A woman aged 66 was stabbed to death by a mugger trying to steal her purse at a bus stop in north London, a witness claimed.
Horrified passers-by heard her shout, “Help me, help me” and then described seeing “a lot of blood”.
Video on social media showed the victim being treated by paramedics near the junction of Limesdale Gardens.
On Friday, Scotland Yard said officers are keeping an open mind regarding a possible motive when asked about robbery.
But Jess Bloom, working at a hair salon opposite, saw the woman stabbed three times by a suspect as he attempted to take her handbag.
Ms Bloom, 24, told The Times: “It happened at the bus stop and the woman ended up in the middle of the road.
“The guy was trying to get her handbag but she said ‘no’ and fought back. I saw her pull the handbag twice. He pulled her into the road and stabbed her three times.
“He let go of her and she dropped to the ground. She was covered in blood and his hand was red. He ran off and everyone ran to the woman to help her. I heard the scream.”
A 22-year-old man arrested in the Colindale area yesterday afternoon remains in custody.
Local resident Salah Ahmed, 35, said he was in a clothes shop near the bus stop when he heard the woman shouting for help.
He then went outside and chased the attacker, who ran away.
Another witness said the assailant left “as cool as a cucumber”, before breaking into a run when people started to chase him.
Mohamed Ble, 47, added: “I saw the woman. She was stabbed in her throat, in her chin, and twice in her chest.
“There was a lot of blood, too much blood. I heard a lot of shouting, I did not know what.
“Then I heard the woman say: ‘Help me, help me’.”
Detective Chief Inspector Alex Gammampila, leading the Met’s Specialist Crime Command investigation, said on Friday: “I know that the effects of this incident will be felt throughout the local community and would like to reassure everyone that a man is in custody and we are not looking for any other suspects.
“We are working hard to establish the full circumstances and motive for this offence which we believe to be an isolated incident.”
DCI Gammampila added: “Burnt Oak Broadway is a busy street for both cars and footfall.
“I am keen to hear from anyone who has phone or dash-cam footage or who may have witnessed something.
“No matter how insignificant it might seem, if you have information please do get in touch.”
Superintendent Tony Bellis from the North West Command Unit, responsible for policing Edgware, said: “A family has been left devastated by this shocking attack and my thoughts are with them at this unimaginable difficult time.
“Local people will notice an increased police presence in the area in the coming days to provide reassurance.
“If you have concerns or just want to speak with officers please do approach them as they patrol.”
The woman’s murder was the capital’s fifth fatal stabbing in ten days.
Jack Hague, 38, was one of three men killed in separate attacks across north and east London during the bank holiday weekend.
A 24-year-old man attended a police station last night has been arrested on suspicion of Mr Hague’s murder. He remains in custody.
Mr Hague’s devastated mother paid tribute, describing him as her “blue-eyed boy”.
Sadiq Khan, elected for a historic third term as London mayor, has vowed to tackle knife crime and said keeping Londoners safe is his “No 1 priority”.
Anyone with information can call the incident room on 020 7175 2206 and 101, quoting 3105/9May, or independent charity Crimestoppers via 0800 555 111.