Singer Brandi Carlile says she also won't participate in a Washington, D.C., conference featuring former Homeland Security Secretary Kirstjen NielsenKirstjen Michele NielsenActing DHS secretary says he expects Russia to attempt to interfere in 2020 elections House Homeland Security rip DHS's 'unacceptable' failure to comply with subpoena Trump puts Kushner in charge of overseeing border wall construction: report 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 TrumpNational Archives says it altered Trump signs, other messages in Women's March photo Dems plan marathon prep for Senate trial, wary of Trump trying to 'game' the process Democratic lawmaker dismisses GOP lawsuit threat: 'Take your letter and shove it' 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 ClintonSupreme Court agrees to hear 'faithless elector' cases Poll: Sanders holds 5-point lead over Buttigieg in New Hampshire Climate 'religion' is fueling Australia's wildfires 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 McSallyPoll: Overwhelming majority say news media making US more politically divided Bill Kristol on McSally calling CNN reporter a liberal hack: 'I guess I'm liberal' McSally dismisses calls to apologize to CNN's Raju for 'liberal hack' comment: 'Called it like it is' MORE (R-Ariz.), Reps. Elise StefanikElise Marie StefanikOvernight Energy: Republicans eye top spot on Natural Resources panel | GOP lawmakers push back on bill to make greener refrigerators, air conditioners | Green groups sue Trump over California fracking plans Republicans push back on bipartisan bill to make greener refrigerators, air conditioners Trump: Pelosi doesn't want to hand over 'fraudulently produced' articles of impeachment MORE (R-N.Y.) and Elissa SlotkinElissa SlotkinHillicon Valley: Biden calls for revoking tech legal shield | DHS chief 'fully expects' Russia to try to interfere in 2020 | Smaller companies testify against Big Tech 'monopoly power' Michigan governor urges Zuckerberg to enforce community guidelines after hate speech, threats surface The Hill's Morning Report — President Trump on trial MORE (D-Mich.), 2020 Democratic presidential candidate Rep. Tulsi GabbardTulsi GabbardPoll: Sanders holds 5-point lead over Buttigieg in New Hampshire Gabbard defeats man in push-up contest at New Hampshire town hall Gabbard on personal meeting with Sanders: 'He showed me the greatest respect' MORE (Hawaii), Anita HillAnita Faye HillSanders campaign official: Biden 'actively courted pro-segregation senators' to block black students from white schools Electability is key to Democrats' 2020 fortunes Clarence Thomas blasts his Biden-led confirmation hearings: 'The idea was to get rid of me' MORE and fashion designer Tory Burch.