U.S. Sen. Amy Klobuchar faced new questions Sunday about the 2003 murder conviction of a black teenager in Minneapolis, an emerging issue that forced her to fend off doubts in a nationally televised interview on Fox News one day ahead of the Iowa presidential caucuses.

In an interview from Des Moines, where she is in the final push of her Iowa campaign, Fox News Sunday host Chris Wallace pressed the former Hennepin County attorney repeatedly on whether she was aware of questionable evidence and tactics cited in a recent Associated Press investigation into the murder conviction of Myon Burrell, who is serving a life sentence in the 2002 shooting death of 11-year-old Tyesha Edwards.

"I didn't know about this new evidence until I saw the report," she said. "I couldn't have. I haven't been in the office for 12 years."

Civil rights activists and black community leaders in Minneapolis have called for Klobuchar to end her presidential campaign until new doubts about the case are resolved, and the issue threatens to deepen as she campaigns in Iowa and beyond.

Wallace noted that Klobuchar has incorporated the Edwards murder as part of her narrative as a tough prosecutor, but that national polls show her registering less than 1% of black support, which could be a problem for her in South Carolina and other states with large African-American populations.

Klobuchar said that Edwards' death deeply shook the African-American community in the Twin Cities and suggested that seeking justice for the girl was a high priority. "It was a tragic case. It was a big deal in the African-American community and our focus was ... doing justice for her family," she said.

Klobuchar sidestepped Wallace's questions about whether prosecutors put the right man behind bars, but suggested she's open to re-examining Burrell's case. She noted that in the past she's worked with the Innocence Project, a nonprofit dedicated to exonerating wrongfully convicted people. "If there is new evidence, it must come forward," she said. "It must be considered immediately by the courts."

The decades-old case has dogged Klobuchar since an AP investigation raised questions about paid witnesses and jailhouse informants used to convict Burrell, who was 16 at the time. He was first prosecuted when Klobuchar was Hennepin County attorney, then tried again and convicted after she left office. On Friday, the jury foreman in the first trial told the AP he regrets voting to convict Burrell and feels jurors were "misled."

Interest in the case has put a spotlight on Klobuchar's record as a prosecutor and highlighted the campaign's struggles to gain support among black voters, a critical Democratic constituency.

Klobuchar defended her standing with black voters in the Sunday interview, noting that she has received the endorsement of two prominent black Iowa legislators.

"It's on me to go across the country and make the case for my agenda, which is very strong when it comes to the African-American community," she said. "It's about economic opportunity, it's about voting rights ... that's on me and I will keep crisscrossing the country, making my case to the African-American community."

Klobuchar campaign aides and some supporters in Iowa downplayed the issue.

"I heard from somebody in Minneapolis last night, otherwise I haven't heard a thing," said Andy McGuire, Klobuchar's Iowa campaign chairman. "That sounds like fishing, and I don't think there's anything on the hook."

Ryan Comstock, a Klobuchar backer in Iowa, said he questioned the timing of the yearlong investigation, which was released just before the start of presidential primary voting. He said he still plans to caucus for her on Monday but acknowledged that concerns about her electability among black voters made it a tougher call to decide between the Minnesota Democrat and former Vice President Joe Biden.