President Donald Trump has spent months mocking Sen. Elizabeth Warren for her claim of Native American heritage, even challenging the Massachusetts Democrat and potential 2020 rival to take a DNA test to prove it.

She did, releasing the results of genetic testing on Monday that says there is "strong evidence" to back up her claim of Native American ancestry. But that didn't satisfy the president, and the war of words between the pair escalated.

"Who cares?" Trump told reporters in response. "DNA test is useless," Trump tweeted a day later, despite once offering to donate $1 million to charity if she took one.

Warren fired back, accusing the president of "racist name-calling."

NBC News fact-checked the various allegations behind the feud, including whether she ever benefited professionally from her self-described minority status, and asked independent experts what the DNA test she took really means.

Here's what we know about Warren's ancestry, and what we don't.

How did the debate over Warren's heritage start?

Warren, who grew up in Oklahoma, land that was once Indian Territory, reportedly listed herself in a directory as a minority professor for nine years (from 1986 to 1995) before eventually landing a job at Harvard.

During Warren's hotly contested Senate bid in Massachusetts in 2012, The Boston Herald uncovered a 1996 student newspaper article quoting a Harvard Law School spokesman who boasted that Warren, who was a professor, was Native American. Her campaign scrambled and failed to offer conclusive proof for the claim, which Warren said stemmed from stories her mother told her as a child.

"Being Native American is part of who our family is and I'm glad to tell anyone about that. I am just very proud of it," Warren told reporters at the time.

In a campaign ad, Warren said she had never asked her mom for documentation that her family was part-Cherokee and part-Delaware, but that it was the reason her parents had to elope.

What do her critics say?

Political rivals over the years have attempted to paint Warren as a liar and an opportunist.

"Elizabeth Warren said she was a Native American, a person of color," then-Sen. Scott Brown, R-Mass., said of Warren during one 2012 debate. "As you can see, she's not."

After Warren emerged as a harsh Trump critic during the 2016 presidential campaign, Trump took to calling her "Pocahontas," after the storied Native American woman.

"She used the fact that she was Native American to advance her career. Elizabeth Warren is a total fraud. I know it. Other people who work with her know it. Elizabeth Warren is a total fraud," the president told NBC News during his presidential bid.

Native American leaders have said the "Pocahontas" nickname is culturally insensitive and racist. Warren herself called it a slur.

As Warren emerged as a likely 2020 contender, Trump offered a million dollars if Warren took a DNA test.

"I will give you a million dollars to your favorite charity, paid for by Trump, if you take the test and it shows you're an Indian," he said at a Montana rally in July. "I have a feeling she will say no."

Did the DNA test she took prove her claims?

Warren had her DNA analyzed by Carlos D. Bustamante, a Stanford University professor and one of the world's leading DNA analysts. He said it was likely she had at least one Native American ancestor some six to 10 generations ago, noting in a campaign-style video released by the senator that five segments of her DNA suggested she was a Native American with "very high confidence."

The test "substantiates" her claim, independent genealogist Megan Smolenyak Smolenyak told NBC News. "Whether anybody accepts it is another matter."

Notably, modern tribes do not accept DNA tests as proof of membership. One prominent native tribe, Cherokee Nation, spoke out against the test. Warren has previously said her family believed they were part-Cherokee and part-Delaware Indian.

"Using a DNA test to lay claim to any connection to the Cherokee Nation or any tribal nation, even vaguely, is inappropriate and wrong," Cherokee Nation Secretary of State Chuck Hoskin Jr. said in a statement. "It makes a mockery out of DNA tests and its legitimate uses while also dishonoring legitimate tribal governments and their citizens, whose ancestors are well documented and whose heritage is proven."

DNA tests are not accepted as proof of tribal citizenship in part because the DNA could not show a specific tribe, only some genetic markers from Native people. What's more, they are more unreliable for Native Americans than for large ancestral regions like Asia or Africa. Many Native Americans also oppose the use of such tests, making the availability of Native American DNA particularly limited.

For this reason, Bustamante used DNA from Mexico and South America, where experts believe relatives of America's Native population also settled some 12,000 years ago, to compare with Warren's DNA.

"The facts suggest that you absolutely have Native American ancestry in your pedigree," Bustamante tells Warren in a video released by the senator along with the results.

What do historians and genealogists say?

Smolenyak, who has traced former first lady Michelle Obama's family back to slaves and former President Barack Obama's ancestors to Ireland, said six to 10 generations back isn't actually that far when it comes to ancestry.

“We’re talking 150-250 years ago," she said. “150 years ago, if there would have been a Native American in her family, would that story have trickled down to her mother? Yeah.”

Native Americans are still the second-largest minority group in Oklahoma, according to the 2010 Census, and tales of Native ancestry are common, experts said.

"My family has a narrative that we're Choctaw, but we don’t have any proof of that," Larry O'Dell, the Oklahoma Historical Society's director of special projects and development, said when asked about tracing Native heritage. "My boss and I were talking earlier, and he has the same kind of narrative. It's just sort of the way it is in Oklahoma."

Smolenyaksaid one of the biggest myths in genealogy is how many families believe their ancestor was a Cherokee princess.

"It's always Cherokee, and it's always a princess," she said.

More than 819,000 people told the 2010 Census they are at least part Cherokee, despite the nation’s three federally recognized Cherokee tribes having only half as many members.

Laura Martin, a deputy director for the Oklahoma Historical Society's research division, said 98 percent of visitors to their library are hunting down ties to Native American heritage. But in order to obtain documentation that's registered by tribes or the U.S. government, ancestors must be listed on historical registers like the Dawes Rolls, which the federal government used to distribute communal tribal land to individual Indians who enrolled from 1898 to 1906.

"We can help you find federal census records, marriage records, death records, things like that, but if they did not sign up [on official documents], it kind of ends there," Martin said.

Did her self-identified minority status help her career?

Warren has adamantly insisted that she never used her heritage to get ahead, and there’s no evidence that it was a deciding factor in her employment at Harvard.

The Boston Globe published an exhaustive investigation in September that included a review of university personnel files — first released to the paper by the senator — and more than 100 interviews with colleagues and every individual who played a role in hiring decisions about Warren who could be reached.

The paper reviewed a University of Pennsylvania document that shows Warren was viewed as a white woman by those who chose to employ her; the hiring committee explains its decision to hire her — over minority candidates — in some length on the form. At Harvard, a human resources form shows that Warren first listed herself as Native American five months after making tenure.

"In sum, it is clear that Warren was viewed as a white woman by the hiring committees at every institution that employed her," the newspaper concluded.

Trump's tweeted claims in October — that Harvard would not have hired her unless she was a minority — are not true.

So, is Warren actually Native American?

Her DNA strongly suggests it.

Warren has been unable to find proof in the historical documents used to confirm such claims by the tribes, however, and she's been quick to note the difference and state that she is not claiming citizenship in any tribe.

"I understand that distinction, but my family history is my family history," she said in a video about the test.

I won't sit quietly for @realDonaldTrump's racism, so I took a test. But DNA & family history has nothing to do with tribal affiliation or citizenship, which is determined only – only – by Tribal Nations. I respect the distinction, & don't list myself as Native in the Senate. — Elizabeth Warren (@elizabethforma) October 15, 2018

Genealogists who have investigated her history have found her relatives to be listed as white in historical documents like the census and do not appear in the Indian documents typically used to verify claims of Native American ancestry, like the Dawes Rolls.

The New England Historic Genealogical Society initially said they had proof that she was 1/32nd Native American, only to later backtrack and say they did not have definitive proof. They declined to comment for this story.

Cherokee genealogist Twila Barnes traced Warren's maternal family back four generations to the turn of the 19th century; the records listed all members as white. She could not find Warren’s family in any of the 45 Indian records and documents she reviewed for a detailed report.

Some Native Americans were not listed in the Dawes Rolls, for instance, and Warren's ancestors could be from well before those records. Still, it does mean that there is no historical documentary proof or evidence that a tribe would accept.

Warren, in a speech this year to a Native American group, addressed the controversy and said she stands by the stories she was told as a child.

"I get why some people think there's hay to be made here. You won’t find my family members on any rolls, and I'm not enrolled in a tribe," she said. But, she added, "my parents were real people."