The Trump administration’s point man for negotiations with the Taliban could soon face a subpoena to testify before Congress about his work as a top lawmaker demands a September hearing.

“I do not consider your testimony at this hearing optional,” House Foreign Affairs Chairman Eliot Engel, a New York Democrat, wrote in a Thursday letter to Zalmay Khalilzad, the State Department’s special representative for Afghanistan reconciliation. “If this letter is insufficient to secure your attendance, I will consider other options that would ensure this hearing takes place in a timely manner.”