Sen. Elizabeth Warren said Tuesday that she was sorry that she identified herself as a Native American for almost two decades, reflecting her ongoing struggle to quiet a controversy that continues to haunt her as she prepares to formally announce a presidential bid.

Her comments more fully explain the regret she expressed last week to the chief of the Cherokee Nation, the first time she’s said she was sorry for claiming American Indian heritage.

The private apology was earlier reported as focusing more narrowly on a DNA test she took to demonstrate her purported heritage, a move that prompted a ferocious backlash even from many allies. Warren will be vying to lead a party that has become far more mindful of nonwhite voters and their objections to misuse of their culture.

“I can’t go back,” Warren said in an interview with The Washington Post. “But I am sorry for furthering confusion on tribal sovereignty and tribal citizenship and harm that resulted.”

Warren has been trying for the past year to get past the lingering controversy over her past assertion that she is Native American.

In addition to the DNA test, she released employment documents over the summer to show she didn’t use ethnicity to further her career. And in a speech a year ago she addressed her decision to call herself a Native American, though she didn’t offer the apology that some wanted at the time.

But as Warren undergoes increased scrutiny as a presidential candidate, additional documents could surface to keep the issue alive.

Using an open records request during a general inquiry, for example, The Post obtained Warren’s registration card for the State Bar of Texas, providing a previously undisclosed example of Warren identifying as an “American Indian.”

Warren filled out the card by hand in neat blue ink and signed it. Dated April 1986, it is the first document to surface showing Warren making the claim in her own handwriting. Her office didn’t dispute its authenticity.

For Warren, putting this chapter behind her is key to calming the nerves of Democrats who want a nominee who can move beyond any problems in their past and present a strong challenge to President Trump.

For the Democratic electorate, roiled by Trump’s racially insensitive comments, it’s become more important for a Democratic standard-bearer to show an understanding of issues related to race and identity.

The nascent 2020 Democratic field is already the most diverse in history, with two black senators, five women, a gay man and an Asian entrepreneur among the announced or potential candidates.

Nonwhite voters have a significant voice in the Democratic primaries. Blacks made up 25 percent of the electorate in the 2016 Democratic primary, according to exit polls. Hispanics made up 7 percent, but that rose to 19 percent in Nevada, a critical early primary state.

It was previously reported that Warren called Bill John Baker, the principal chief of the Cherokee Nation, and apologized for sharing the results of a DNA test which showed she had a distant relative who was Native American.

Julie Hubbard, a spokeswoman for the Cherokee Nation, declined to address the scope of the conversation between Warren and the chief.

Warren, asked in a brief interview Tuesday if she’d intended the apology to include labeling herself as Native American when at the University of Pennsylvania and at Harvard University, replied “yes.” She gave the same response when asked if it included labeling herself as a minority in the Association of American Law Schools directory.

“I told him I was sorry for furthering confusion about tribal citizenship,” Warren said. “I am also sorry for not being more mindful about this decades ago. We had a good conversation.” CNN reported her broader apology on Monday.

The apology has met with mixed reactions. Several tribal members applauded her. “This closes the matter,” tweeted Keith Harper, a citizen of the Cherokee Nation and a former U.S. ambassador to the U.N. Human Rights Council. “Onward.”

But not all were pleased.

“I want to see it in writing,” said David Cornsilk, a historian and genealogist who is also a citizen of the Cherokee Nation. “I want her to go on national TV. I want her to do a video like she did to announce her DNA results. It just seemed very lacking.”

In October, Warren released the DNA results showing she had a Native American ancestor six to 10 generations ago. The move backfired, with Cherokee leaders outraged that she used the test to show any connection to the tribe, a process they control. It also dredged up uncomfortable issues about defining race via bloodlines.

The test was an attempt to quell the criticism — and occasional mockery — she’s faced for years. President Trump has frequently called her “Pocahontas.” Allies of former Massachusetts senator Scott Brown, whom Warren ousted in 2012, referred to her as “Fauxcahontas.”

Republicans have charged that Warren claimed Indian ancestry to advance her career, though an investigation by the Boston Globe showed that Warren was considered a white woman when hired as a law professor by the University of Pennsylvania and then by Harvard University.

The Texas bar registration card is significant, among other reasons, because it removes any doubt that Warren directly claimed the identity. In other instances Warren has declined to say whether she or an assistant filled out forms.

The card shows her name, her gender and the address for the University of Texas law school in Austin, where she was working at the time.

On the line for “race,” Warren neatly printed, “American Indian.” She left blank lines for “National Origin” and “Physical handicap” and signed the document.

“She is sorry that she was not more mindful of this earlier in her career,” said Kristen Orthman, a Warren campaign spokeswoman.

Warren filled out the card after being admitted to the Texas bar. Warren was doing legal work on the side, but nothing that required bar admission in the state, according to her campaign.

The date coincided with her first listing as a “minority” by the Association of American Law Schools. Warren reported herself as minority in the directory every year starting in 1986 — when AALS first included a list of minority law professors — to 1995, when her name dropped off the list.

Warren also had her ethnicity changed from white to Native American in December 1989 while working at the University of Pennsylvania. The change came two years after she was hired there.

Several months after Warren started working at Harvard Law School in 1995, she okayed listing her ethnicity as Native American. Harvard listed Warren as Native American in its federal affirmative action forms from 1995 to 2004, records show.

There’s no indication Warren had anything to gain by reporting herself as Native American on the Texas bar card. Above the lines for race, national origin and handicap status, the card says, “The following information is for statistical purposes only and will not be disclosed to any person or organization without the express written consent of the attorney.”

Separately, Warren’s candidate financial disclosure report was released Tuesday showing that she and her husband have a net worth between $5.2 million and $9.1 million, in line with other reports of her wealth.

Warren, who made a career in academia with research on how debt leads to personal bankruptcy, reported no mortgage on homes in Cambridge, Mass., and Washington. The couple also reported no credit card debt.

Anu Narayanswamy contributed to this report.