In an interview with NPR aired Monday, author Michael Wolff defended the claim in his book that the office of the special counsel headed by Robert S. Mueller III considered indicting President Donald Trump.

In his new book, Siege: Under Fire, Wolff claims that he obtained a document from the Mueller investigation that included in it the possibility of Trump’s indictment. The document, which he calls a “draft indictment,” outlines various legal arguments that the special counsel could use if Trump were indicted and challenged Mueller’s ability to do so. “It assumes that the president has been indicted,” Wolff told NPR. “It assumes that the president has gone into court and made a motion to dismiss the indictment on the grounds that a president cannot be indicted. And this is the response to that motion.” However, the office of the special counsel has denied that any such document exists, and is sticking to the claim that Mueller considered it unconstitutional to indict a sitting president.

While Wolff refused to identify his source, he attempted to use his previous book, Fire and Fury: Inside the Trump White House, as evidence of his legitimacy. “I’ve written one book that has been, I think, largely confirmed by all subsequent accounts. So I’m pretty familiar with, if not extremely familiar with, everybody I’m talking to here. And then I like to hear it a couple of times, and in the situation of people I trust of hearing things more than once, and then it gets into the book.”

Siege, which was released on Tuesday, was one of the most anticipated books of the summer, partially due to the success of Wolff’s previous book. Fire and Fury, provided an intimate and humiliating look into how Trump acted in the white house, often painting him as a baby who had to be taken care of by the White House Chief of Staff.