Linda Tripp, whose tape recordings of Monica Lewinsky triggered a criminal investigation of President Clinton, was subpoenaed to testify before the grand jury next week, one of Tripp's lawyers said.
Word of her appearance, scheduled for Tuesday, comes as Whitewater prosecutor Kenneth Starr tries to negotiate an agreement for the cooperation of Lewinsky, the former White House intern who alleged that she had a presidential affair and cover-up in conversations with Tripp.
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Starr spokesman Charles Bakaly declined to comment.
Meanwhile, Starr's prosecutors questioned Senior Presidential Adviser Sidney Blumenthal before the grand jury about his discussions inside the White House with First Lady Hillary Clinton.
It was Blumenthal's third grand jury appearance. He had refused to answer some earlier questions on the grounds of executive privilege.
After spending most of the afternoon before the panel, Blumenthal told reporters, "If Starr is interested in the truth, he heard it today."
He also said: "I have answered the questions put to me truthfully. The president shared his account of the Lewinsky matter with me unguardedly and freely under the assumption that we were speaking in complete privacy.
"What I told the grand jury under oath supports what the president has told the American people and is contrary to any charge that the president has done anything wrong."
In biting comments, his lawyer, William McDaniel, said the questions put to Blumenthal "were sophomoric, silly, insulting to the grand jury, and insulting to this process and certainly insulting to the president."
The grand jury is investigating whether Clinton had a sexual relationship with Lewinsky and then urged her to lie about it.
Tripp, a former White House employee who moved to the Pentagon, became friends with Lewinsky when the intern was transferred to the Pentagon in the spring of 1996.
By Pete Yost