WASHINGTON (AP) — House Democrats have promoted Republican Rep. Liz Cheney to vice chairwoman of a committee investigating the Jan. 6 Capitol insurrection, placing her in a leadership spot on the panel as some Republicans are threatening to oust her from the GOP conference for participating.

Cheney, a fierce critic of former President Donald Trump, has remained defiant amid the criticism from her own party, insisting that Congress must probe the Capitol attack, in which hundreds of Trump's supporters violently pushed past police, broke into the building and interrupted the certification of Joe Biden's presidential election victory.

"We owe it to the American people to investigate everything that led up to, and transpired on, January 6th," Cheney, R-Wyo., said in a statement as Democrats announced her promotion on Thursday. "We will not be deterred by threats or attempted obstruction and we will not rest until our task is complete."

Cheney's appointment as vice chairwoman comes amid an effort by some Republicans to oust Cheney and Rep. Adam Kinzinger of Illinois from the GOP conference because they accepted their appointments to the panel from House Speaker Nancy Pelosi. A draft letter by Arizona Rep. Andy Biggs to Republican Leader Kevin McCarthy obtained by The Associated Press calls Cheney and Kinzinger "two spies for the Democrats" whom Republicans cannot trust to attend their private meetings.

Biggs, chairman of the conservative House Freedom Caucus, is calling on the conference to change its rules to state that any Republican who accepts a committee assignment or serves on a committee without a recommendation from GOP leadership "shall immediately cease to be a Member of the Conference." McCarthy's office did not respond to a request for comment about Biggs' proposal.

Cheney, who was already booted from her position as GOP conference chairwoman earlier this year, has been undeterred by the criticism, despite serious primary challenges back home. The daughter of former Vice President Dick Cheney has formed the unlikely alliance with Pelosi in what she has framed as an existential fight for the party and for democracy itself.

"Every member of this committee is dedicated to conducting a non-partisan, professional, and thorough investigation of all the relevant facts regarding January 6th and the threat to our Constitution we faced that day," Cheney said in the statement. "I have accepted the position of Vice Chair of the committee to assure that we achieve that goal."

As the committee has met privately, Cheney has worked closely with Democrats in determining the direction of the probe. The committee's chairman, Mississippi Rep. Bennie Thompson, said in the statement announcing Cheney's appointment that Democrats are "fortunate to have a partner of such strength and courage" and that Cheney's insights have shaped the early work of the panel.

Cheney "has demonstrated again and again her commitment to getting answers about January 6th, ensuring accountability, and doing whatever it takes to protect democracy for the American people," Thompson said.

The vice chair position, usually reserved for a member of the Democratic majority, gives Cheney a top role on the panel after McCarthy decided not to appoint any of his members to the committee. McCarthy pulled all five Republicans he had chosen after Pelosi rejected two of them, and he has harshly criticized Cheney and Kinzinger for participating at her request.

The committee's work is just getting started and could last months or years. Thompson issued broad requests for information last week to law enforcement agencies and social media companies about the planning of the insurrection, and this week he asked technology and telecommunications platforms to preserve personal communications surrounding the attack.

In July, the panel held an emotional first hearing with four police officers who battled the insurrectionists and were injured and verbally abused as the rioters broke into the building and repeated Trump's lies about widespread election fraud.

At the hearing, Cheney expressed to the officers "deep gratitude for what you did to save us" and defended her decision to accept an appointment on the committee.

"The question for every one of us who serves in Congress, for every elected official across this great nation, indeed, for every American is this," she said then. "Will we adhere to the rule of law, respect the rulings of our courts, and preserve the peaceful transition of power?"