The U.S. House of Representatives Foreign Affairs Committee chairman said on Monday he signed a subpoena to be delivered to Secretary of State Antony Blinken for documents related to the August 2021 U.S. withdrawal from Afghanistan.

Michael McCaul has launched an investigation into the messy withdrawal from Afghanistan under Democratic President Joe Biden and events in the country since.

Republicans – and some Democrats – say there has never been a full accounting of the chaotic operation, in which 13 U.S. service members were killed at Kabul’s airport, Reuters reported.

McCaul had given the State Department until Monday to produce the documents.

“Unfortunately, Secretary Blinken has refused to provide the Dissent Cable and his response to the cable, forcing me to issue my first subpoena as chairman of this committee,” McCaul said in a statement.

He said the subpoena would be delivered on Tuesday morning.

About two dozen U.S. diplomats in Afghanistan sent a confidential cable through a so-called dissent channel warning Blinken in July 2021 of the potential fall of Kabul to the Taliban as U.S. troops withdrew from the country, The Wall Street Journal reported in 2021.

Blinken said during a hearing last week that the department had already shared information and was working to provide more, but that some specific details could only be shared with senior officials, a move intended to protect the identity of those who had expressed dissent.

Asked by Reuters for comment on the subpoena, the State Department referred to remarks by spokesperson Vedant Patel at Monday’s press briefing. Patel said it was “vital to us that we preserve the integrity” of the dissent channel. He said the department was “prepared to make the relevant information in the cable available through briefings or some other mechanisms.”