Excerpts from a book manuscript by former national security adviser John Bolton leaked over the weekend just as the book went on presale, sending shockwaves through Washington in the midst of the Senate impeachment trial of President Trump.

In a bombshell story, the New York Times reported that in a manuscript that’s been circulating around Washington, Bolton claims that Trump told him he wanted to deny foreign aid to Ukraine unless the country agreed to investigate former Vice President Joe Biden and his son Hunter. Trump has accused the younger Biden of corruption tied to his work with Ukraine-based company Burisma Holdings.

On Twitter, Trump disputed Bolton’s claims, writing, “I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens…If John Bolton said this, it was only to sell a book.”

Trump has ordered Bolton not to testify at his impeachment trial, but the Senate could still subpoena him.

Bolton’s book, for which he received a reported $2 million advance, went on presale over the weekend. Publisher Simon Schuster describes the memoir as “a substantive and factual account of his time in the room where it happened.”

The book, titled The Room Where It Happened: A White House Memoir, is slated for publication on March 17.

Michael Schaub is an Austin, Texas–based journalist and regular contributor to NPR.