Sources familiar with the White House legal strategy said Starr contacted the president's personal attorney, David Kendall, by phone to tell him the subpoena had been issued and that the subpoena was delivered to Kendall's office shortly afterward.

Starr's phone call triggered urgent negotiations currently underway over how the president could provide information to the grand jury without making a personal appearance before the panel, where witnesses cannot be accompanied by counsel, the sources said.

Starr is investigating whether the president and Lewinsky, a former White House intern, had a sexual relationship and then lied about it under oath. The sources said it now appears likely that Clinton will provide testimony in some form.

The subpoena presents a profound political and legal dilemma for the president. Challenging the subpoena could result in a political nightmare and spark a constitutional crisis. - CNN