Questions over Elizabeth Warren's claims of Native American heritage were revived by her rival for U.S. Senate Monday in a television ad characterizing Warren as untrustworthy.

Sen. Scott Brown, locked in a tight battle with Warren in Massachusetts, also used the topic to attack Warren at a debate last week. He asserts Warren used Native American heritage to help her chances of being hired by Harvard Law School as a professor.

In the 30-second spot, news clips focusing on Warren's claims are used to cast doubt on the former Obama administration official's trustworthiness. In the final clip, a reporter asks Warren if there is anything that will "come out about you that we don't already know."

"I don't think so, but who knows," Warren responds.

Last week, Brown opened an hour-long debate with the issue, claiming Warren wasn't being totally transparent about why she was listed at Native American when she was a professor at Harvard Law School.

"Professor Warren claimed that she was a Native American, a person of color and as you can see, she's not," Brown said. He repeated his call for his opponent to release records showing whether she had claimed Native American heritage on job applications.

"When you are a U.S. Senator, you have to pass a test and that's one of character and honesty and truthfulness. I believe and others believe she's failed that test," Brown said.

Warren defended her claim, saying that she had learned of her heritage from her family, adding that "my mother was part Delaware and part Cherokee."

Warren has denied that she ever used her heritage to gain access to college or employment.

Previously, Warren has said that she listed herself as having Native American roots while teaching at Harvard University Law School to meet other people with similar backgrounds.