A pensioner suffered a heart attack after going to help a woman who was being harassed by a man on London Underground.
Police have launched an appeal for a man they want to speak to about the incident, in which a 43-year-old woman was verbally abused when her phone started ringing.
A 65-year-old man stepped in and was then menaced by the abusive passenger himself.
When the train terminated at Uxbridge, the pensioner, from Wembley, suffered a heart attack. He was given CPR by the woman and was treated in hospital.
The incident took place on 3 January after the train departed its previous station, Eastcote.
Police have issued an image of a man they wish to contact (pictured above), as well as another man who may have filmed the incident on his mobile phone.
DC Holmes added: "I urgently want to speak to this man, as we believe he might have significant information about what happened. Do you recognise him? If you do, please get in touch with BTP.
"I would also like to talk to anyone who was on the train and witnessed the incident, and has not yet come forward. In particular I would like to trace a young man – possibly Asian – who was seen to film the incident on a mobile phone. The footage could be important evidence that we would like to view."
Anyone who has information can call British Transport Police on 0800 40 50 40.