US Senator and prospective Democratic candidate for President Elizabeth Warren listed her race as "American Indian" on a State Bar of Texas registration card in 1986, the media reported.
The yellow registration card, which is dated April 1986, was filled out in blue ink and signed by Warren, the Washington Post reported on Tuesday. The card listed Warren's alma maters the University of Houston and Rutgers Law School but no other racial identifiers.
A Warren aide did not dispute the card's authenticity and declined to comment on whether there might be other unreported examples of her having claimed American Indian heritage.
Elizabeth Warren has been trying to get past the lingering controversy over her past assertion that she is a Native American.
In an interview with the Post, Warren said she was sorry for claiming American Indian heritage.
According to the daily, Warren had previously declined to answer whether she or an assistant filled out forms in which her race was listed as Native American.
Elizabeth Warren released a video last year that included a DNA test of her heritage. But this sparked a ferocious criticism from Native Americans about whether she was attempting to circumvent the tribal citizenship process.
News of the card came following Warren's acknowledgment earlier this week that she had apologized to Cherokee leaders for causing "confusion" by her use of a DNA test to prove Native American ancestry.
"I'm not a tribal citizen and I respect the difference," the Massachusetts Democrat had told CNN on Monday. "Tribes and only tribes determine tribal citizenship."
The prospective 2020 Democratic presidential candidate is expected to formally announce her entrance into the race on Saturday.
"I can't go back. But I am sorry for furthering confusion on tribal sovereignty and tribal citizenship and harm that resulted," she told the Post.
"As Senator Warren has said she is not a citizen of any tribe and only tribes determine tribal citizenship. She is sorry that she was not more mindful of this earlier in her career," said her spokesperson Kristen Orthman in a statement.
But Republican National Committee spokesman Mike Reed accused Warren of apologizing at a time that was politically convenient. "Now, four days before her formal presidential launch, she's issued a politically opportunistic apology that doesn't go nearly far enough. Warren pretended to be a minority to climb the Ivy League ladder -- a lie that will continue to haunt her presidential ambitions."