The devastated father and brother of a man stabbed to death during a mugging spoke of their grief and anger yesterday as they appealed for information to catch his killers.

John Tanner, a 53-year-old chartered surveyor from Blakedown, Worcestershire, and his eldest son John David Tanner, aged 26, could barely speak as they told of their anguish at the murder 23-year-old Paul Tanner as he walked through the graveyard of St Paul's church in Birmingham's Jewellery Quarter.

Paul and a friend were returning from a night out when they were mugged by three youths at knifepoint. He was stabbed in the chest and died in hospital last Friday.

His father said: "I sat for 20 hours watching the life drain from my beautiful son. I cannot believe we are here. We sit here on our own because his mum and his younger brother Chris just cannot be here.

"He was 23, he was at the beginning of his life, he loved life, and he was part of a loving family. He had started a job that he loved, he had a great circle of friends, and then somebody chose to take his life.

"This was not part of a fight or battle. He was not putting up any resistance, this person just stabbed him. There must be people out there who know who did this.

"No good will come out of it, no good can come out of it. These people must be found, they have got to be found, so please come forward." Police confirmed they were still looking for the killer.

Four youths from the Birmingham area, aged between 15 to 18, were arrested on Sunday. Three of them were released without charge and the other has been released on police bail pending further inquiries.

Detective Inspector John Pyke, of the murder investigations unit, revealed that Paul was mugged ten minutes after another mugging in the area, which is believed to have been carried out by the same gang.

In that incident, a young couple were robbed of their money at knifepoint on Caroline Street and escaped shaken but uninjured.

Mr Pyke appealed to anyone in the area who might have seen anything suspicious to come forward.

He said: "I'm convinced that there are people out there who will know the identity of these people, whether it be family or friends, and I urge them to talk to the police and I urge members of the public to come forward and give us the identity of these people.

"There are a lot of taxis that pick up people in and around St Paul's so I urge taxi drivers to cast their minds back to see if they can remember these individuals.

"Clearly, because of their age, their behaviour would have changed dramatically since the stabbing and then the realisation that Paul Tanner had died."

The Rev Tom Pyke, vicar of St Paul's Church in the Jewellery Quarter, has appealed for people to come forward and lay flowers of protest in the churchyard.

Friends, colleagues and family have laid flowers at the spot where Paul was attacked and Rev Pyke asked the city's young people to lay flowers to signal their unwillingness to have their city dominated by violent crime.

He said: "Paul's death is the waste of a promising young life. If we are to see any good come in the aftermath of this tragedy, then it has to be communities standing together and sending a clear signal that we will not tolerate the carrying of knives or guns."

A book of condolence has also been opened at the church.

Anyone with information is asked to contact police on 0121 626 4017 or Crimestoppers on 0800 555111.