Former national security adviser John Bolton’s team on Friday accused the White House of trying to suppress his unpublished book after word surfaced that the National Security Council had sent him a letter expressing concerns that the manuscript contained classified information.

The White House has been scrutinizing Bolton’s book, which is due out next month, for classified materials as part of a standard pre-publication review process.

Sarah Tinsley, a senior adviser to Bolton, issued a statement Friday acknowledging that a letter from the NSC had been sent to Chuck Cooper, Bolton’s lawyer. News that the letter had been sent had started circulating in Washington earlier in the day.

“This latest leak from the NSC’s pre-publication review process raises even more serious concerns that the process has been thoroughly breached, and that it is more about suppressing Ambassador Bolton’s book than about classification issues,” Tinsley said.

She said Bolton was reviewing the letter and would respond. She said Bolton continues to believe “that the manuscript contains no legitimately classified material.”

The book alleges President Donald Trump tied the suspension of military aid to Ukraine to the country’s willingness to help with investigations into the president’s political rivals. That allegation would contradict a key defense that Trump’s lawyers asserted at his Senate impeachment trial — that there was no quid pro quo when it came to aid to Ukraine. Trump was acquitted Wednesday on charges of abuse of power and obstruction of Congress.

Revelations about Bolton’s book emerged during Trump’s trial. Democrats demanded that he be summoned to testify, but Republicans rejected calls to ask him to testify.