Sen. Elizabeth Warren, D-Mass., on Wednesday was once again forced to apologize for claiming Native American ancestry on a 1986 registration card for the Texas state bar, and left the door open that there may be more documents out there with a similar claim.

The controversy appears to be difficult for the senator to put behind her. She had been planning to formally launch her presidential campaign on Saturday. A reporter on Wednesday asked if she would drop out of the race and she responded, "Thank you."

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Fox News' Shepard Smith called Warren's actions "cultural appropriation." A Boston Globe columnist wrote flatly: With latest revelation, Elizabeth Warren can't beat Donald Trump.

Questions about Warren’s heritage date to at least 2012, when her Republican opponent seized on the issue during her first Senate campaign to wrongly argue she identified as a Native American to advance her career. President Donald Trump frequently deploys a racial slur to criticize Warren.

Warren, who apologized last Friday to the Cherokee Nation for revealing the results of a DNA test last autumn that showed just a trace amount of Native American lineage, was asked if there are any other documents where she claimed the ancestry.

"So all I know is during this time period, this is consistent with what I did because it was based on my understanding from my family's stories," she said. "But family stories are not the same as tribal citizenship."

The registration card was first reported by The Washington Post. Fox News has verified the document, which marks the first known instance of Warren claiming Native American ancestry on an official document.

Hillary Chabot, a reporter at The Boston Herald, wrote that the senator is looking to distance herself from the scandal, but brought up the wealth issue as another potential headache for Warren.

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"Warren has worked hard to craft a narrative as a scrappy Dust Bowl native ready to take down vast corporations and billionaires like Starbucks CEO Howard Schultz. But she usually fails to mention her own vast wealth," she wrote, pointing out that Warren and her husband are worth as much as $11 million.

Paulette Jordan, a former Democratic state representative in Idaho and a member of the Coeur d’Alene Tribe, told The Associated Press, “It’s not exactly how you’d want to enter the arena” as a presidential candidate.

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She apologized in private last week to the principal chief of the Cherokee Nation for “causing confusion on tribal sovereignty and tribal citizenship and the harm that resulted,” said tribal spokeswoman Julie Hubbard.

“I am not a tribal citizen. Tribes, and only tribes, determine citizenship,” Warren said, adding, “I have apologized for not being more sensitive to that distinction. It’s an important distinction.”

Fox News' Paul Steinhauser and the Associated Press contributed to this report