The Washington Post has confirmed that Mark Felt, a former top FBI official, was Deep Throat, the source who leaked secrets during the Watergate scandal.
Mark Felt says he only told his secret to his family three years ago
The scandal forced the resignation of Republican President Richard Nixon in August 1974.
Deep Throat helped Washington Post reporters Bob Woodward and Carl Bernstein uncover the Watergate affair.
Mr Felt's family said he deserved recognition for the risks he took - but some in the US called him a "traitor".
Mr Felt's role was revealed by US magazine Vanity Fair after three decades of mystery and speculation.
Initially the Washington Post reporters refused to confirm Mr Felt's identity, sticking with their 31-year promise only to break the silence after their source's death.
But later on Tuesday, they issued a joint statement, saying: "Mark Felt was Deep Throat and helped us immeasurably in our Watergate coverage.
"However, as the record shows, many other sources and officials assisted us and other reporters for the hundreds of stories that were written in The Washington Post about Watergate."
Mr Felt, now 91, told Vanity Fair: "I'm the guy they used to call Deep Throat."
The name derived from a famous pornographic film of the time.
Mr Felt only admitted his secret to his family in 2002, he told the magazine, when his daughter confronted him after being tipped off by one of his close associates.
The FBI has not commented on the admission.
In the 1970s, Mr Felt was convicted of organising illegal searches of houses of radicals associated with the Weather Underground movement.
He was pardoned by President Ronald Reagan in 1981.
After the revelations, Mr Felt's grandson, Nick Jones, described his grandfather as "a great American hero who went well and above the call of duty at much risk to himself".
Some US newspapers said he showed the value of whistleblowers in keeping the government in check.
But former Nixon aides condemned him.
G Gordon Liddy, who was jailed for four and a half years for his role in the Watergate break-in, said Mr Felt "violated the ethics of the law enforcement profession".
Nixon's speechwriter and later US presidential candidate Pat Buchanan called Mr Felt a "traitor".
When Nixon resigned in August 1974, it was the first time any US president had done so.
The Watergate scandal concerned a break-in at the offices of the rival Democratic party in the Watergate building in Washington in 1972.
The attempted bugging of the building was linked to officials in the Nixon White House, and the cover-up went all the way to the top.
The reporters' role in the affair was immortalised in the 1976 film All The President's Men.
During their investigation, Deep Throat assisted Woodward and Bernstein with prompts and hints.
If Woodward needed to meet the source to check information, he would place a flag in a flower pot on a certain place on his window sill, as a signal for the pair to meet in secret in an underground car park in the dead of night.
For decades, there had been speculation about who the source was - but no credible individual had ever come forward.