LONDON (AP) _ Police launched an investigation Wednesday into a mail bomb explosion that killed a 32-year-old man at his home in northeast England.

Stephen Hoggarth of Newbridge Hill died of severe chest injuries when the bomb exploded Saturday, minutes after a postman delivered the device in a package, Lincolnshire police said.

Police initially thought Hoggarth was killed in a gas explosion, and only Wednesday confirmed it was a mail bomb.

Hoggarth's 3-year-old son, Callum, was visiting him at the time, but was not injured in the blast.

Police said the bomb appeared to be a grudge attack and not part of a larger campaign.

Authorities said the postman delivered the package to Hoggarth's apartment at 8:30 a.m. Saturday, and neighbors said they heard a loud noise shortly thereafter.

No one called the police, however, and the blast was not reported until Hoggarth's relatives tried to contact him several hours later. Hoggarth's son then answered the phone, saying his father was dead.

Hoggarth worked as a floor-layer and his boss, Howard Parkinson, said he ``was well-liked and very jovial. No one can understand why this has happened.''

Detective Chief Superintendent Chris Cook said about 50 officers have been assigned to the investigation.

``Whoever did this was callous, calculating and irresponsible,'' he said.