Police are looking for two “heroic” skateboarders who intervened when a man was fatally stabbed in the middle of Oxford Circus.
A 60-year-old man was attacked shortly before 8pm on Thursday in Oxford Circus in central London.
He was rushed to hospital where he died later in the evening as a result of his injuries.
Tedi Fanta Hagos, 25, from Swansea has since been charged with his murder.
Two men, who were pictured carrying skateboards, immediately stepped in to try and intervene as the 60-year-old was attacked, police said.
One of the men used his skateboard to pin down the suspect before police arrived and arrested him.
Hagos did not sustain any lasting injuries as he was held down, police said.
After the emergency services arrived, the two skateboarders left the scene without speaking to police officers.
Watch: Teenager stabbed to death in Sydenham
Scotland Yard said the extent of the role they played in apprehending the suspect came to light after detectives trawled through CCTV and other video footage of the incident.
Detective Chief Inspector Vicky Tunstall, who is leading the investigation, said: “In addition to being immensely brave, we believe these two individuals may also be key witnesses.
“In stepping forward as they did to apprehend a violent individual, they showed great courage and potentially prevented others from being attacked.
“I want to be clear that while they used force to subdue the attacker, those actions were entirely reasonable and are not in question.
“They can be assured that we wish to speak to them solely as witnesses.”
The pair are urged to call the incident room on 0208 785 8099 or get in touch with detectives online using this form.
Watch: Inquest finds police believed Fishmongers' Hall jihadi was trying to trigger bomb