US: In 1975, a CIA officer in Greece, Richard Welch, was killed after former colleague Philip Agee outed him in a book. Since then, it has become a crime for US federal employees to knowingly disclose the identity of a CIA agent.
This explains why a claim that White House officials knowingly leaked the name of an undercover CIA agent in revenge for her husband's criticism of President George Bush has suddenly grown into a full-blown Washington scandal.
Democrats called yesterday for an independent council to conduct an inquiry after it emerged at the weekend that the CIA had asked the Justice Department to investigate the possibility of White House involvement.
White House spokesman Mr Scott McClellan yesterday denied widespread rumours that Mr Bush's top political adviser, Mr Karl Rove, or anyone else in the White House, was behind the leak. He said he had spoken to Mr Rove about the allegations and was assured that it was "simply not true" that he had anything to do with it.
The agent outed was Ms Valerie Plame, wife of former US ambassador Joseph Wilson, who had publicly discredited the case made by Mr Bush in his January State of the Union address, citing British intelligence, that Saddam Hussein was trying to buy uranium in Niger.
The controversy has echoes of the scandal in Britain over the leaking by aides to Prime Minister Mr Tony Blair of the name of government scientist Dr David Kelly as the source of a BBC story discrediting London's case for war against Iraq.
Syndicated columnist Robert Novak first reported the leak on July 14th, citing "two senior administration officials" and giving Mrs Wilson's maiden name, Palme, which she used overseas as a covert officer.
But only now has the finger been pointed directly at the White House, after the Washington Post reported that officials in the White House had called as many as six reporters to leak the name.
Mr Wilson wrote in the New York Times, eight days before Mr Novak's column appeared, that he had gone to Niger in February 2002 at the request of the CIA and found the uranium claim to be without foundation.
He accused the Bush administration of "misrepresenting the facts on an issue that was a fundamental justification for going to war". His article forced the White House to admit Mr Bush should not have included the claim in the speech.
On Sunday, the Washington Post, quoting an administration official, said the identity of Mr Wilson's wife was leaked to discredit him by making it appear he was only sent to Africa because of her.
CIA officials said yesterday that she had nothing to do with the decision to send Mr Wilson to Niger.
In interviews and and speeches throughout the summer, Mr Wilson complained about the leak, which he said showed that "if you talk, we'll take your family and drag them through the mud".
Senator Charles Schummer of New York demanded an investigation and New York Times columnist Paul Krugman called it a "criminal act", but the story disappeared from the front pages.
Senator Schumer has now been joined by several colleagues in calling for an independent inquiry as the scandal assumes mini-Watergate proportions with the White House going into defensive mode.
"Leaking the name of a CIA agent is a horrible thing to do - tantamount to putting a gun to that agent's head," he said.
On top of that, "such information can jeopardise the security of this nation".
Mr Wilson yesterday told ABC News that he did not have any knowledge that Karl Rove authorised the leak "but I have great confidence that, at a minimum, he condoned it and certainly did nothing to shut it down".
He said that the White House was "acting like a schoolyard bully, pulling the hair of a little girl".
Valerie Plame was an undercover CIA operative specialising in weapons of mass destruction. Intelligence analysts pointed out that exposing her name endangered people who co-operated with her, and could allow foreign intelligence services to track down some of her former contacts and lead to the exposure of agents.
Under the Intelligence Identities and Protection Act, it is a criminal act punishable by up to 10 years in prison to identify a CIA operative without authorisation.
All the leading Democratic candidates called for an independent council to conduct the investigation rather than the Justice Department under attorney general John Ashcroft.
Mr Agee, who outed the agent in Greece in 1975, is currently living in exile in Cuba.