A former News of the World journalist who made phone-hacking allegations against the paper has been found dead at his home in Watford.
Mr Hoare had told the New York Times hacking was far more extensive than the paper acknowledged when police first investigated hacking claims.
Sean Hoare also told the BBC's Panorama phone hacking was "endemic" at the NoW.
A police spokesman said the death was currently being treated as unexplained, but was not thought to be suspicious.
Meanwhile computer hackers have tampered with the website of The Sun, which is also owned by News International.
Readers were briefly directed to a hoax story which said Rupert Murdoch has been found dead in his garden.
A group of hackers called Lulz Security, which has previously targeted games companies and US government websites, claimed responsibility via Twitter.
Visitors to the Sun website were redirected to the group's Twitter page.
News International said it was "aware" of what was happening but made no further comment.
Earlier Metropolitan Police Assistant Commissioner John Yates resigned after growing pressure amid the phone-hacking scandal.
It has also emerged that a former senior News of the World (NoW) journalist carried out work for Scotland Yard dealing with witnesses and suspects while employed by the paper.
Alex Marunchak was employed by the Met as a Ukrainian language interpreter and was on Scotland Yard's list of interpreters between 1980 and 2000.
According to the BBC's Panorama, the former NoW Irish edition editor obtained e-mails hacked into by a private detective in 2006. Mr Marunchak denied receiving "any unlawfully obtained material".
In a statement, the Met said it would look into the matter, saying: "We recognise that this may cause concern and that some professions may be incompatible with the role of an interpreter."
Hertfordshire Police said Mr Hoare's body was discovered after police were called to his home in Langley Road, Watford at 1040 BST on Monday.
They said: "The death is currently being treated as unexplained, but not thought to be suspicious. Police investigations into this incident are ongoing."
When he first spoke out, Mr Hoare told Panorama the then NoW editor Andy Coulson had asked him to hack phones - something Mr Coulson has denied.
Earlier, Met Police Assistant Commissioner John Yates resigned after he was informed he would be suspended pending an inquiry into his links with a former NoW journalist.
Mr Yates had checked the credentials of the paper's former deputy editor Neil Wallis before he too was employed by the Met.
Mr Wallis was arrested and released on bail on Thursday on suspicion of conspiring to intercept communications.
Met Commissioner Sir Paul Stephenson, the most senior policeman in Britain, resigned on Sunday after also facing criticism for the force's recruitment of Mr Wallis as a PR consultant.
Mayor of London Boris Johnson said Deputy Commissioner Tim Godwin would be in charge at Scotland Yard until Sir Paul's replacement was appointed. Mr Yates will be replaced in the interim as the Met's head of counter-terrorism by Assistant Commissioner Cressida Dick.
Mr Johnson said it was right for both Sir Paul and Mr Yates to stand down. Mr Yates said his conscience was clear and had "deep regret" over his resignation.
The IPCC said four referrals relating to the police's phone-hacking investigation involved Commissioner Sir Paul Stephenson, who resigned on Sunday, and Mr Yates, as well as two other former senior officers.
The BBC understands the other two officers are former Assistant Commissioner Andy Hayman and former Deputy Assistant Commissioner Peter Clarke.
A fifth referral relates to the alleged involvement of Mr Yates in inappropriately securing a job at the Met for the daughter of a friend.
The BBC understands the woman to be Amy Wallis, daughter of Mr Wallis, and she works in a civilian non-operational role.
In the Commons, Home Secretary Theresa May announced Her Majesty's Inspectorate of Constabulary would look into corruption in the police, and an Independent Police Complaints Commission investigation on the same issue would be part of the judge-led inquiry into the hacking scandal.