Ms Linda Tripp testified yesterday before the secret grand jury investigating President Clinton's relationship with former White House intern Ms Monica Lewinsky. Ms Tripp, a public relations official at the Pentagon, recorded 20 hours of private conversations with Ms Lewinsky.
She denied to the Washington Post yesterday that she made the tapes to get a book contract.
She met Ms Lewinsky after the latter was transferred against her will from the White House to the Defence Department.
The older woman befriended the younger, who boasted that she had had a relationship with Mr Clinton. Unknown to Ms Lewinsky, Ms Tripp was taping every word.
In yesterday's Washington Post interview Ms Tripp refused to discuss the 20 hours of tapes, or what they contain, or her grand jury testimony which will continue today and, if necessary, tomorrow.
She is an essential witness to the investigation of events she neither witnessed nor knew anything about, other than what Ms Lewinsky told her in confidence.
Ms Tripp's lawyer listened in on the Washington Post interview and counselled his client on questions to answer and not to answer.
Her testimony is crucial for the public prosecutor, Mr Kenneth Starr. Without it there is no evidence that Mr Clinton did anything untoward in his relations with Ms Lewinsky other than to befriend a young woman with stars in her eyes, awed by the President's interest in her.
The grand jury will want to know if Ms Lewinsky had sexual relations with the President of the US, which Mr Starr will probably need to establish to justify his charges.
Otherwise he is dealing in gossip and Ms Tripp, with a somewhat unsavoury reputation, is not only his key, but his only, witness.
"I am so happy to go before the grand jury and tell the truth," Ms Tripp told the Washington Post. She gave her secret tapes to Mr Starr, she said, because she was worried for her country by what Ms Lewinsky told her.
It has been alleged that she was writing a book about the Clinton presidency at the time.
She complained of unfair criticism for betraying the friendship of a young woman she herself had befriended.
She has spent over 100 hours rehearsing what she will say to the grand jury and how she will say it.
The concern in Congress is not so much about what happened, but whether Mr Clinton lied about it to the public and obstructed justice to cover it up.
There was more to the Watergate scandal than a cover-up, but President Richard Nixon was driven from office because his White House covered up his crimes and lied about them.
In the Paula Jones case, when Mr Clinton was questioned about his relations with Ms Lewinsky, he denied in a deposition last January under oath that he had sexual relations with the former White House intern.
Ms Lewinsky, who has a new set of lawyers, reportedly is negotiating with Mr Starr's office.
After seven hours Ms Tripp left the courthouse without commenting to the news media. However her lawyer, Mr Anthony Zaccagnini, told reporters that Ms Tripp said she "found it very easy to truthfully answer the questions posed to me by the prosecutor and the grand jury".