Senator Elizabeth Warren said on Tuesday she was sorry 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 has 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. Ms Warren is on the cusp of formally announcing her presidential bid and will be vying to lead a party that has become far more mindful of non-white voters and their objections to misuse of their culture.

"I can't go back," Ms 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."

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

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In addition to the DNA test, she released employment documents over the summer to show she did not 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 did not offer the apology that some wanted at the time.

But as Ms 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 Ms Warren's registration card for the State Bar of Texas, providing a previously undisclosed example of Ms Warren identifying as an "American Indian."

Ms Warren filled out the card by hand and signed it. Dated April 1986, it is the first document to surface showing Ms Warren making the claim in her own handwriting. Her office did not dispute its authenticity.

For Ms 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 his or her past and present a strong challenge to President Trump.

For the Democratic electorate, roiled by Mr Trump's racially insensitive comments, it has 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.

Non-white voters have a significant voice in the Democratic primaries. Black people made up 25 per cent of the electorate in the 2016 Democratic primary, according to exit polls. Hispanic voters made up 7 per cent, but that rose to 19 per cent in Nevada, a critical early primary state.

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

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

Ms Warren, when asked in a brief interview on Tuesday whether she had intended the apology to include labelling herself as Native American when at the University of Pennsylvania and at Harvard University, replied "yes." She gave the same response when asked whether it included labelling herself as a minority in the Association of American Law Schools directory.

"I told him I was sorry for furthering confusion about tribal citizenship," Ms 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.

Several tribal members applauded her. "This closes the matter," tweeted Keith Harper, a citizen of the Cherokee Nation and a former US ambassador to the United Nations 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."

Elizabeth Warren takes major step towards running for president in 2020

In October, Ms 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 has faced for years. President Donald Trump has frequently called her "Pocahontas." The campaign of former senator Scott Brown, R-Mass, whom Ms Warren ousted in 2012, referred to her as "Fauxcahontas."

Republicans have charged that Ms Warren claimed Indian ancestry to advance her career, though an investigation by the Boston Globe showed Ms 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 Ms Warren directly claimed the identity. In other instances, Ms 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," Ms 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 spokesperson.

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

The date coincided with her first listing as a "minority" by the Association of American Law Schools. Ms 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.

Ms 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 Ms Warren started working at Harvard law school in 1995, she approved listing her ethnicity as Native American. Harvard listed Ms Warren as Native American in its federal affirmative action forms from 1995 to 2004, records show.

There is no indication Ms 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 organisation without the express written consent of the attorney."

Separately, Ms Warren's candidate financial disclosure report was released on 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.

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