WASHINGTON, D.C.: Independent Whitewater prosecutor Kenneth Starr has subpoenaed Hillary Rodham Clinton, the White House disclosed Monday. She will testify this Friday in Washington before a federal grand jury regarding the mystery surrounding the discovery of her Whitewater billing records. "Friday's testimony will offer the First Lady the opportunity to tell the independent counsel what she knows about these matters," the White House said in a statement. Mrs. Clinton has testified under oath four times previously about Whitewater, but this will be her first appearance before the Whitewater grand jury. "It's serious," notes TIME's James Carney. "In addition to highlighting Starr's investigation, this subpoena strengthens the hand of Senate Whitewater chairman Alfonse D'Amato, who will soon seek to extend the Senate investigation past the original February 29th deadline." Earlier Monday, D'Amato rebuffed Hillary Clinton's offer to provide written responses to questions about her involvement in the Whitewater affair. The Committee "looks forward to hearing Mrs. Clinton's responses," he replied, but not before the White House provides further information such as White House e-mail messages. Longtime Clinton associate Carolyn Huber described to the Whitewater Committee last week how she accidentally and unaccountably discovered Hillary Clinton's Whitewater billing records, missing under subpoena for 18 months, in a room in the Clinton residence. That testimony has elevated the controversy to a new level, notes TIME's Viveca Novak. "It's an inherently intriguing mystery that has captured the public's interest," she says.