During her opening statement in Thursday’s impeachment hearing, Fiona Hill, the former top White House adviser on Russia, said that she believed individuals who possess valuable information related to the Ukraine scandal have a “moral obligation” to come forward. For many, the line was taken as a subtle jab at the various Trump administration officials who have spurned requests for their testimony, including Hill’s former boss, John Bolton.

So far, Bolton has refused to testify without a court order—a legal process that could prolong the impeachment investigation. Still, his lawyer has publicly suggested that he has juicy details to share. In the meantime, Bolton has inked a lucrative book deal, sent oddly cryptic Tweets, and returned to his political action committee, Bolton PAC.

Asked about the former national security adviser’s antics on Sunday, House Intelligence Committee chair Adam Schiff (D-Calif.) didn’t hide his displeasure. “He was invited to come in, and he did not choose to come in and testify,” Schiff told CNN’s Jake Tapper. “His deputy, Fiona Hill, and his other deputy, Col. Vindman, and Tim Morrison, others in the national security council have shown the courage to come in.”

Adam Schiff on John Bolton's refusal to testify before Congress: "He will have to explain, one day, if he maintains that position why he wanted to wait to put it in a book instead of telling the American people what he knew when it really mattered to the country" #CNNSOTU pic.twitter.com/W7cN6zhMtE — CNN Politics (@CNNPolitics) November 24, 2019

“He will have to explain one day,” Schiff continued, “if he maintains that position why he wanted to wait to put it in a book instead of telling the American people what he knew when it really mattered to the country.”

Schiff then pointed to Hill’s defiance of White House efforts to block her testimony as the kind of “courage” Bolton lacks. “She made the decision that this is the right thing to do. John Bolton should make the same decision.“

Tapper noted that even if Bolton refuses to appear before the House, he might still be compelled to testify in a Senate trial. “He could,” Schiff responded, before adding, “That doesn’t relieve him of the obligation right now to show the courage that Dr. Hill did.”