Hoare, 47, accused his former editor, Andy Coulson, of being aware of the illegal activity. Hoare was found dead at his home only days after making a series of fresh allegations against his former superiors.
Police called the death "unexplained," but declined to comment on whether foul play was suspected.
David Cameron, Britain's Prime Minister, cut his trip to Africa short to fly home after new developments in the case.
Cameron hired Coulson as his Downing Street media strategist after the former editor quit News of the World.
Hoare worked for the Sun and the News of the World until 2005 and was brought front and center into the phone hacking scandal last year when he became the first reporter to go on the record with allegations that Coulson knew about the hacking and "actively encouraged" his staff to intercept voicemail messages.
Hoare told the New York Times that Coulson's claims of ignorance regarding the phone hacking were "simply a lie," according to the Daily Telegraph. He later told the BBC that his former editor had personally asked him to access phone messages, another claim that Coulson denied.
Hoare told a journalist from the Guardian last week that "There's more to come. This is not going to go away."
Hoare had received treatment in the past for drink and drug problems, and recently had badly injured his foot in an accident.
