Detectives want to speak to a woman who helped at the scene of a fatal crash on the A4 Great West Road in Osterley earlier this year.
Janusz Adam Partyka was killed in a collision with a blue BMW, which failed to stop, near the junction with Wood Lane on Sunday February 8.
The 31-year-old pedestrian, who lived locally and was engaged to be married, died at the scene, with a post-mortem giving the cause of death as a head injury.
Police this week said they were trying to trace a woman known only as 'Jan' who had stopped her vehicle at the scene and called for emergency services.
She said at the time she was on her way to pick up her parents from the airport when she witnessed the crash.
She returned to the spot some days later and left a condolence card, which police said had touched the hearts of Mr Partyka's family.
Detective Sergeant Jeff Edwards, of the Serious Collision Investigation Unit, said: "I am appealing for this woman to call me, since she may have important information to assist with this most serious of enquiries.
"If you know anyone, possibly named Jan, who has mentioned witnessing the incident, please would you urge her to contact police."
The collision took place on the westbound carriageway of the A4, and police were called just before 5am.
A 28-year-old man was arrested the following day on suspicion of causing death by dangerous driving. He remains on police bail.
Anyone with information is asked to call the Serious Collision Investigation Unit at Merton on 020 8543 5157. To remain anonymous, call Crimestoppers on 0800 555 111, quoting reference 1401661/15.