Former Homeland Security Secretary Kirstjen NielsenKirstjen Michele NielsenThis week: House kicks off public phase of impeachment inquiry Senators urge Trump to fill vacancies at DHS Trump taps Chad Wolf as new acting DHS secretary MORE says she resigned from her post earlier this year because "it became clear that saying no” to policies she disagreed with “was not going to be enough."

Nielsen, the top official in charge of implementing President TrumpDonald John TrumpThis week: House kicks off public phase of impeachment inquiry Impeachment week: Trump probe hits crucial point Judd Gregg: The big, big and bigger problem MORE's hard-line immigration policies, left the administration in April.

"What led me to resign is there were a lot of things — that there were those in the administration who thought that we should do," Nielsen said during an often-combative interview at Fortune magazine's "Most Powerful Women" summit in Washington on Tuesday.

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"Just as I spoke truth to power in the very beginning," Nielsen continued, "it became clear that saying no and refusing to do it myself was not going to be enough.”

When pressed by "PBS NewsHour's" Amna Nawaz if she regretted signing a memo that resulted in families being separated at the U.S.-Mexico border, Nielsen replied, "I don't regret enforcing the law because I took an oath to do that, as did everybody at the Department of Homeland Security."

"We don't make the laws," Nielsen, 47, said.

The Trump administration faced widespread scrutiny last year for a policy that led to the separations of thousands of families. Trump later signed an executive order to end the policy, though thousands of families reportedly remain separated.

"What I do wish had worked a lot better is that the coordination and information flow are simply insufficient for that number of people coming," said Nielsen, who still works in an adviser role for the Trump administration. "It's heartbreaking that any family felt at any time that they had to cross the border illegally."

Nielsen's appearance at the Fortune conference was not without controversy. A day earlier, singer Brandi Carlile announced she was dropping out of the event in protest of Nielsen's involvement.

"I don't think that human rights violators and merit-based abusers of displaced people should be given a platform to 'reimagine' history," Carlile said in a Twitter post.

Former Democratic presidential nominee Hillary ClintonHillary Diane Rodham Clinton3 ways government can help clean up Twitter Intelligence Democrat: Stop using 'quid pro quo' to describe Trump allegations The Memo: Bloomberg's 2020 moves draw ire from Democrats MORE, who was also poised to speak at the women's summit, also reportedly bowed out because of Nielsen.

Before Nielsen spoke, Fortune Editor-in-Chief Clifton Leaf acknowledged the controversy.

"These conversations can be tough, and arguably should be tough," Leaf told the audience. But, he said, part of the magazine's mission "is to provide a platform for these discussions."