Former President Donald Trump instructed four associates to defy subpoenas issued late last month from the House Select Committee investigating the Jan. 6 Capitol attack, according to multiple outlets.

The documents requested by the committee were due Thursday, and the four associates among them former Trump adviser Steve Bannon and former White House chief of staff Mark Meadows were instructed to testify before the committee on Oct. 14 and 15.

Politico and The Washington Post reported Thursday that they had reviewed a copy of a letter Trump sent through his attorney. In it, Trump reportedly signaled that he intends to sue in order to block the subpoenas directed at Bannon, Meadows, former White House social media chief Dan Scavino and former Defense Department official and House Intelligence Committee aide Kash Patel.

All four witnesses were either working in the Trump White House or communicating with the Trump administration in the days leading up to the violence at the U.S. Capitol.

In his letters to each of them, Trump reportedly cited executive privilege, the somewhat vaguely defined concept that presidents have the right to withhold information and communications that might be of interest to the public or Congress. It is not mentioned in the Constitution, but courts have generally supported the idea that subjective opinions and advice are protected under executive privilege. Otherwise, everyone who confers with the president in private might have to worry about their words becoming public.

In court, executive privilege is generally assessed on a case-by-case basis.

President Trump is prepared to defend these fundamental privileges in court, the letters said, per Politico.

It is not clear how the committee, or the Department of Justice, will respond if its subpoenas go unanswered.

Select Committee Chairman Rep. Bennie Thompson (D-Miss.) wrote in each of his Sept. 23 letters to the witnesses that the panel is investigating the facts, circumstances, and causes of the January 6th attack and issues relating to the peaceful transfer of power, in order to identify and evaluate lessons learned and to recommend to the House and its relevant committees corrective laws, policies, procedures, rules, or regulations.

Thompsons committee has already taken steps to get its hands on communications from the Trump White House relating to the attack. In August, it sent a sweeping records request to the National Archives, which handles presidential records, and to several federal agencies asking for documentation.

White House press secretary Jen Psaki indicated last month that President Joe Biden was unlikely to block any Trump-era records from making their way onto committee members desks, but the administration later issued a clarification saying it would evaluate such requests individually.