The chairman of the oversight panel leading the Benghazi investigation threatened Tuesday to subpoena Americans wounded in the terror attack.

Rep. Darrell Issa (R-Calif.) wrote to Secretary of State John Kerry, giving him until Sept. 24 to make the witnesses available. The State Department has said it wouldn’t force recalcitrant employees to talk to lawmakers but Issa pointed out that several have talked to media outlets and were interviewed by the State Department’s independent panel investigation Benghazi last year.

“I must receive confirmation that the Department will make these witnesses available to Committee investigators by September 24, 2013,” the letter states. “Otherwise, I will have no alternative but to consider the use of compulsory process.”

“The survivors of the attacks are the only people who can give testimony to the Committee about what happened on the ground in Benghazi,” Issa wrote. “Details provided by the survivors will not only help the Committee determine what took place during the attack, but will also help the Committee and other interested parties determine ways to prevent future tragedies.”