A 62-year-old man collapsed and died after confronting a group of youths he caught breaking into a neighbour's car.
James Barrett collapsed after chasing after the gang of youths
James Barrett was babysitting his grandchildren when he heard the gang break the window of a car outside his house in Anfield, Liverpool.
He ran out to chase the youths away and followed them down Bingley Road, but collapsed in nearby Priory Road.
He was taken to the Royal Liverpool Hospital where he died just before midnight on Friday.
Merseyside Police are hunting the gang of about five youths and said a full investigation was under way.
It is not yet clear whether any of the youths struck Mr Barrett.
But Det Ch Supt Peter Currie, leading the inquiry, said one of the youths was seen to "run by him from behind".
He said there was no obvious injuries on Mr Barett's body, but a post-mortem examination would be carried out on Saturday.
His family said: "We are completely shocked and devastated by this.
"Jim was a wonderful man who would do anything for anyone.
"As well as being a wonderful father, husband and granddad he was a true Liverpool supporter, with a real love of football.
"He will be greatly missed by all his family and many friends, who will remember him for his kindness, generosity and warmth."
Mr Barrett leaves a wife, Mary, and two sons who live in the local area.
Mr Currie said Mr Barrett was just doing the "neighbourly thing" when he went to investigate the smashing glass.
"There was a group of youths outside. It looked like they threw a brick through the car window," he added.
Mr Barrett pursued the youths for about 100 yards before he collapsed, Mr Barrie added.
Merseyside Police said the scene was cordoned off after the incident for a detailed forensic investigation. They are treating the case as a major inquiry.
A spokeswoman said five or more youths were involved, all aged between 13 and 15. They were wearing tracksuits and some were in hooded tops.
Officers are appealing for information from anyone who saw the incident or might know the identity of the youths involved.