Speculation is mounting over a potential court fight on former national security adviser John Bolton’s testimony in a Senate impeachment trial—but what would a legal challenge to Bolton’s upcoming book look like?

In a Jan. 23 letter obtained by several media outlets, Ellen Knight, the senior director for records, access and information security management at the National Security Council, told Bolton’s attorney Charles Cooper of Cooper & Kirk that after a “preliminary review, the manuscript appears to contain significant amounts of classified information.”