The former president said that the letter was a prelude to arrest and indictment.
That trial, centering on whether he falsified business records, is scheduled to start in March.
He also faces several civil trials.

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The first contest, the Iowa caucus, is scheduled for January 15.
In her setting the schedule, Cannon noted the complexities in the case and the volume of documents involved.
The trial is scheduled to take place in Fort Pierce, FL.

Cannon, the judge, was a Trump appointee.