Washington — Three days before The New York Times revealed details of a manuscript by former national security adviser John Bolton, the National Security Council (NSC) warned his attorney that his book contained classified and top secret information that could not be published or disclosed publicly.

Bolton, according to The Times, claims in his forthcoming book that the president linked Ukraine aid and investigations into the Bidens. Senators will soon decide whether to call Bolton and other witnesses to testify in the ongoing impeachment trial. The White House's position has long been that it would assert executive privilege to keep Bolton from divulging sensitive information.

Bolton submitted the manuscript to the NSC on December 30 for a standard prepublication review. In a letter dated January 23 and released on Wednesday, NSC senior director Ellen Knight told Bolton's attorney Charles Cooper that the manuscript "appears to contain significant amounts of classified information," based on a preliminary review.

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"It also appears that some of this classified information is at the TOP SECRET level, which is defined by Executive Order 13526 as information that 'reasonably could be expected to cause exceptionally grave harm to the national security of the United States if disclosed without authorization,'" Knight wrote. "Under federal law and the nondisclosure agreements your client signed as a condition for gaining access to classified information, the manuscript may not be published or otherwise disclosed without the deletion of this classified information."

A senior White House official on Wednesday stressed that the letter was in no way a "threat" to Bolton, saying that characterization would be a "gross mischaracterization at best and a flat out lie at worst." The official noted the NSC offered to work with Bolton to clear the manuscript for publication.

News of the letter comes on the first day of senator questioning in the impeachment trial.

Bolton, a meticulous notetaker, has been working with and identifying sensitive and classified material for years. Bolton's book, "The Room Where It Happened: A White House Memoir," is set to be published in March by Simon & Schuster, which is a subsidiary of ViacomCBS.

This is a developing story and will be updated.