Monday's surprise grand jury witness in the New York "hush-money" investigation into Donald Trump is Giuliani lawyer Robert Costello, a source within the defense confirmed to Insider.
A possible indictment for Trump has been on hold until Costello can testify before a Manhattan grand jury on Monday. Costello is a former legal advisor to Michael Cohen, a key witness for the Manhattan District Attorney's office.
Cohen told Insider he will be available as a potential rebuttal witness.
Trump called on his supporters to "protest" and "take our nation back" on Saturday in a post on Truth Social after he predicted his own potential indictment before the grand jury.
The grand jury, which is between 16 to 23 members in number, could reach a vote by the end of the three-hour session on Monday, but that is unlikely, experts previously told Insider.
This is a developing story.