BOSTON (AP) – The U.S. Senate campaign pitting Massachusetts Republican Sen. Scott Brown and likely Democratic challenger Elizabeth Warren has taken an unexpected turn by delving into whether Warren has claimed Native American heritage in her academic career.

Warren, who grew up in Oklahoma, said she’s proud of her family ties to Cherokee and Delaware tribes – a heritage she said she learned through stories passed down to her from older family members.

At the same time, Warren has said she wasn’t aware officials at Harvard Law School had promoted her as a Native American faculty member in the 1990s, even though academic directories from 1986-1995 indicated Warren had identified herself as a “minority law teacher” before being hired by Harvard.

Warren hasn’t talked about her Native American heritage on the campaign trail up until now.

Warren’s campaign has yet to produce any documentation of her Native American ties, although they say they are looking. Warren also told reporters that she couldn’t recall using her heritage to claim a minority status when seeking a job.

Brown’s campaign manager, Jim Barnett, said the story “raises serious questions about Elizabeth Warren’s credibility.”

“Prof. Warren needs to come clean about her motivations for making these claims and explain the contradictions between her rhetoric and the record,” Barnett said.

Warren’s campaign accused Brown of using smears to call into question “the qualifications and ability of a woman.”

“If Scott Brown has questions about Elizabeth Warren’s well-known qualifications ... he ought to ask them directly instead of hiding behind the nasty insinuations of his campaign and trying to score political points,” said Warren’s campaign manager, Mindy Myers.

Brown, speaking briefly to reporters on Monday, and said it’s up to Warren to answer questions raised by the media.

“If there are questions, she should answer them just like we’ve been asking and answering questions about our taxes,” he said, referring to the candidates’ recent release of their income tax returns. “If you’re in this position and you’re asked to be transparent, then you should do so.”

The professor who recruited Warren to Harvard said that any suggestion that she got her job in part because of a claim of minority status is wrong.

“That’s totally stupid, ignorant, uninformed and simply wrong,” Harvard Law School professor Charles Fried said Monday. “I presented her case to the faculty. I did not mention her Native American connection because I did not know about it.”

The story first surfaced last week when the Boston Herald found a 1996 article in Harvard’s student newspaper, the Harvard Crimson, about student dissatisfaction about the number of women and minority professors on the Harvard Law faculty. In the story, Harvard Law spokesman Mike Chmura referred to Warren as Native American.

Warren said on Friday that she was unaware Harvard had promoted her as a minority professor.

Law school directories from the Association of American Law Schools from 1986 to 1995 put Warren on the association’s list of “minority law teachers” when she was teaching at the University of Texas and the University of Pennsylvania.

According to the directories, the list is made up of “those legal educators who stated they were members of a minority group.”

Warren’s campaign said she was told by older family members that her grandmother and grandfather on her mother’s side could trace their lineage back to the Cherokee and Delaware tribes.

“She learned about her heritage the way most Americans learn about their heritage, from conversations with her parents, her grandparents, her aunts and uncles,” said Warren campaign aide Kyle Sullivan.

Michael Dean with the Oklahoma Historical Society said it’s not unusual for families from Oklahoma to claim some Native American heritage.

“There was so much intermarriage back in the 1890s that was fairly common,” Dean said.

Tara Damron, assistant curator of the society’s American Indian collection, said finding a definitive answer about Native American heritage can be difficult, not only because of intermarriage, but also because some Native Americans opted not to be put on federal rolls, while others who were not Native American did put their names on rolls to get access to land.

“There are a lot of people in Oklahoma who do haveNative lineage but can’t prove that,” Damron said.