The alarm was raised by three-year-old Callum, who told his grandfather on the telephone that "daddy", Stephen Hoggarth, was dead.
Detectives were last night investigating the private life of Mr Hoggarth, 32, for any clues to the identity of his killer. They have ruled out a terrorist attack and are examining possible links with former lovers of Mr Hoggarth's girlfriend, with whom he lived. The woman was not in the house at the time of the explosion.
Mr Hoggarth died from huge injuries to his chest after the parcel bomb exploded in the kitchen of his flat in Newbridge Hill, in the market town of Louth, Lincolnshire.
According to neighbours, the floor layer moved into the flat six weeks ago after "hurriedly" leaving his old home, where he had taped up the letter box with industrial-strength plastic. "We thought it was a bit strange," said one yesterday.
The police said that they believed the murderer was someone with a grudge.
The explosion, which severed Mr Hoggarth's hands, occurred on Saturday morning, but was at first thought to have been an accident involving a gas blast.
Lincolnshire police announced yesterday they had launched a murder inquiry.
Mr Hoggarth died at about 8.30am, shortly after the parcel bomb was delivered. Callum was in another room of the flat when the device detonated.
The boy had been due to visit his grandparents and Mr Hoggarth's father telephoned the flat at about 10.30am when they failed to arrive.
Mr Hoggarth's two sisters and one of their husbands then went to the flat and discovered the body.
Last night Callum was staying with his mother, Lynn, Mr Hoggarth's estranged wife.
Detective Chief Superintendent Chris Cook said yesterday: "Whoever did this was callous, calculating and irresponsible. The device would have been handled by numerous Post Office staff prior to delivery and it is only a miracle that Callum was not killed as well."
Police believe the bomber had a good knowledge of explosives because the device was so effective.
Forensic experts will spend the next few days trying to obtain clues from the remains of the bomb.Reuse content