WASHINGTON (AP) — Donald Trump intends to assert executive privilege in a congressional investigation into the Jan. 6 insurrection at the Capitol, a move that could prevent the testimony of onetime aides, according to a letter sent by lawyers for the former president.

The letter went to at least some witnesses who were subpoenaed by the House committee, and it makes clear that Trump plans to invoke privileges meant to protect presidential communications from being shared with Congress. The letter’s substance was described Thursday by a person who has seen it and spoke on condition of anonymity to The Associated Press because the letter was not yet public.

Spokespeople for Trump did not immediately return messages seeking comment.

The move sets the stage for a likely clash with House Democrats who are investigating the roles of Trump and his allies in the run-up to the riot, when thousands of Trump supporters broke into the Capitol as Congress was certifying the results of the presidential election won by Democrat Joe Biden.

Last month, the committee, which was formed over the summer, issued subpoenas to Mark Meadows, Trump’s former chief of staff; Dan Scavino, the former deputy chief of staff for communications; Kashyap Patel, a former Defense Department official; and Steve Bannon, a former Trump adviser.

The committee has issued more than a dozen subpoenas to people linked to plans for the Jan. 6 protests, including three additional witnesses announced Thursday. Those individuals would be less likely to get help from Trump’s executive privilege claims, which would be limited to people who worked in the White House.

Politico reported the existence of the letter earlier Thursday, the deadline the committee set for compliance.

It was not immediately clear how witnesses would respond to Trump’s decision to invoke executive privilege or what consequences they would face if they refuse to cooperate.