Singer Brandi Carlile says she also won't participate in a Washington, D.C., conference featuring former Homeland Security Secretary Kirstjen NielsenKirstjen Michele NielsenChad Wolf becomes acting DHS secretary Senate paves way for Trump's next DHS chief Divided Supreme Court leans toward allowing Trump to end DACA MORE.

The "Joke" singer was poised to perform at Fortune's Most Powerful Women summit on Tuesday. But on Monday, Carlile said she was nixing her appearance because Nielsen was one of the featured speakers.

"I don't think that human rights violators and merit-based abusers of displaced people should be given a platform to 'reimagine' history," Carlile wrote to her more than 100,000 followers on Twitter.

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"The atrocity of family separation at our Southern border needs to go down in history as one of the United States of America's most merciless acts," the Grammy Award-winner wrote.

While noting it's "rarely her style" to drop out of "such a special appearance," Carlile said, "Respectfully, I absolutely cannot support Kirstjen Nielsen having a voice among the most powerful and inspiring women in America."

President TrumpDonald John TrumpGOP senators balk at lengthy impeachment trial Warren goes local in race to build 2020 movement 2020 Democrats make play for veterans' votes MORE announced in April that Nielsen, the top official in charge of implementing his hard-line immigration policies, would leave her post.

Nielsen is scheduled to be interviewed at Fortune's conference by "PBS NewsHour's" Amna Nawaz for a conversation titled, "The Hard Questions." According to the conference's website, she'll address "the horror of family separation, border security, and more."

The move by Carlile comes days after former Secretary of State Hillary ClintonHillary Diane Rodham Clinton2020 Democrats make play for veterans' votes The Memo: Democrats confront prospect of long primary Manafort sought to hurt Clinton 2016 campaign efforts in key states: NYT MORE reportedly opted out of the event, citing a scheduling conflict. But an unidentified source told Slate that the former Democratic presidential nominee chose to withdraw from the conference upon word that Nielsen was also slated to appear.

Last month, Nielsen bowed out of a speaking event hosted by The Atlantic following backlash from progressive groups.

Among those also on the docket for Fortune's conference: Sen. Martha McSallyMartha Elizabeth McSallyThis week: House kicks off public phase of impeachment inquiry Progressive veterans group launches campaign labeling Trump as a 'national security threat' Advocates step up efforts for horse racing reform bill after more deaths MORE (R-Ariz.), Reps. Elise StefanikElise Marie StefanikMeadows slams ex-Bush aide's 'reprehensible' remark about GOP congresswoman GOP motions to subpoena whistleblower Live coverage: House holds first public impeachment hearing MORE (R-N.Y.) and Elissa SlotkinElissa SlotkinOvernight Health Care: Walden won't seek reelection | Senate Dems to vote this week to overturn Trump ObamaCare moves | Largest children's migrant shelter to close | Vulnerable Republicans balk at drug pricing bill House passes third bill aimed at preventing foreign election interference Avoiding the snake in the grass: Let's not allow impeachment to divide us MORE (D-Mich.), 2020 Democratic presidential candidate Rep. Tulsi GabbardTulsi GabbardThe Hill's Morning Report - Diplomats kick off public evidence about Trump, Ukraine Outsider candidates outpoll insider candidates Krystal Ball: Tulsi Gabbard surges, is she the most electable? MORE (Hawaii), Anita HillAnita Faye HillFive landmark moments of testimony to Congress Christine Blasey Ford makes rare public appearance to accept empowerment award Anita Hill: 'I am ready to hold Joe Biden accountable' MORE and fashion designer Tory Burch.