Martin Jacks, 23, a trainee with Ernst and Young, bled to death in his ground-floor maisonette in Putney, south-west London, on Saturday afternoon from a large wound in his thigh inflicted by a 12-bore shotgun fired at close range.
Police said that Mr Jacks, a Rugby school scholarship pupil and Cambridge natural sciences graduate, was of unblemished character. There was no obvious motive for the murder.
Mr Jacks's background was being investigated last night but detectives said it was unlikely that he had been involved in anything dubious such as drugs dealing.
He had moved to London two years ago and was due to take his final accountancy examinations next month. He shared the flat with a friend from Cambridge, who was out at the time of the attack, and had a girlfriend.
The possibility that it was a burglary that went wrong was largely discounted. There was no evidence of property being stolen and Mr Jacks's wallet and credit cards were untouched. It is rare for burglars of such properties to be armed.
The three men, who were wearing balaclavas, arrived at about 4.30pm while Mr Jacks was watching the Test match on television and broke down the door to gain entry.
He was shot within seconds and died within three minutes, police said.
The men left the estate, of mixed private and municipal housing, in a dark-coloured car, described by eyewitnesses as 'sporty'. A spent shotgun cartridge was found in the kitchen and three live cartridges outside the maisonette.
Detective Superintendent Chris Burke, who is leading the investigation, said: 'The motive for this is a complete mystery.
'This murder took place during the middle of the afternoon, with people out sunbathing on the grass between the flats. Three masked men bursting into homes like that just doesn't happen that often around here.'
One possibility being examined was that Mr Jacks was the victim of a mistaken identity killing or, on the basis of where he was struck, an attempt to warn or intimidate someone. Detectives were checking on previous owners and occupants of the flat.
His father, Brian Jacks, 47, a project manager from Rugby, Warwickshire, told a press conference yesterday that he knew no reason why anyone would want to kill his son.
He said: 'We are a very close family. Martin was liked and respected by all his peers. We are very proud of both our sons. Our family is absolutely devastated.'
Martin's mother Pat, 47, and his brother Stuart, 20, were too upset to attend the press conference. Mr Jacks and Mr Burke appealed for anyone with information to come forward.
Robin Alden, his housemaster at Rugby, described Mr Jacks as 'a most pleasant, rather self-effacing, young man'.