Democratic presidential contender Elizabeth Warren told the Native American Issues Forum she made “mistakes” in how she handled past claims of Native American ancestry, which critics have long said gave her an unfair advantage in applications for law school professorships.

The Massachusetts senator on Monday expressed regret over her handling of claims to tribal heritage during the first day of the Frank LaMere Native American Presidential Forum in Sioux City, Iowa, touted as the first venue for White House hopefuls to sell their candidacies to Native American Democrats. Her decision to attend the event comes on the heels of last week’s release of her plan addressing native communities.