Former US President Donald Trump's former chief of staff, Mark Meadows, has filed a lawsuit against the Committee of Inquiry into the storming of the Capitol.
The lawsuit, filed on Wednesday, questions the legality of the "unreasonably incriminating" subpoena before the committee.
The MPs had previously threatened Meadows with criminal action if he did not appear in person before the panel.
The lawsuit is against the Democratic Speaker of the House, Nancy Pelosi, and the nine members of the committee, including Chairs Bennie Thompson (Democrats) and Liz Cheney (Republicans).
After it had previously looked as if Meadows would cooperate in the parliamentary investigation, a legal tug-of-war is now threatening.
Trump's key witness originally wanted to testify
Meadows is believed to be a key witness to Trump's role in efforts to prevent Joe Biden from being elected.
He had initially resisted a summons to testify before the committee, but then agreed to appear in person - before changing his decision again.
In response to his recent U-turn, the committee wrote to Meadows late Tuesday that it had "no choice" but to bring him to justice for disregard for the House of Representatives.
With his lawsuit, Meadows came before the committee.
Radical Trump supporters stormed the Capitol on January 6th, when the victory of Democrat Joe Biden in the presidential election on November 3rd was to be certified there.
The attack, which made headlines around the world, killed five people.
The investigative committee set up by the House of Representatives is supposed to clarify the exact background of the attack.
The panel has summoned a number of former Trump employees - including Trump's former chief strategist Steve Bannon.
He had refused to testify before the committee.
The House of Representatives then initiated legal action against him - this is now also threatening Meadows.
If convicted, Meadows could face up to 12 months in prison, but a fine is more likely.