What does "principle witness" mean?

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In the case of Scooter Libby, there is no suggestion that there was any payment or improper inducement to win a pardon. Rather, the allegation by critics is that the pardon was granted to send a message to Trump’s associates – including his besieged personal lawyer in New York, Michael Cohen – not to cooperate with federal prosecutors.

The White House denies sending any implied message to targets of investigations. “Pardoning Libby was the right thing to do after the principle witness recanted her testimony,” White House Press Secretary Sarah Sanders told members of the media.

But Ms. Sanders has refused to rule out a possible future pardon for Mr. Cohen. “It is hard to close the door on something that hasn’t taken place,” she said Monday. “I don’t like to discuss or comment on hypothetical situations that may or may not ever happen.”