ABC News/screengrab
ABC News' David Muir interviews former Secretary of State Hillary Clinton.
"That was a mistake. I'm sorry about that. I take responsibility," Clinton said, according to a brief snippet of an ABC News interview released before it airs in full later in the day.
The full context of her remarks was not immediately available, but ABC reported that her statement "is the farthest Clinton has gone yet in offering an apology for her use of a private email server."
Clinton, the Democratic front-runner in 2016, exclusively used a personal email server for her State Department work between 2009 and 2013.
Her critics accuse her of breaking protocol, trying to dodge government disclosure laws, and jeopardizing sensitive information an insecure system.
Clinton has repeatedly insisted what she did was allowed and that she did not send or receive anything that was marked classified at the time. Some of her communications are being retroactively classified as they are slowly released to the public.
Despite Clinton's defense, the entire controversy - including her decision to delete thousands of emails her team deemed personal - has appeared to damage her popularity and cause voters to increasingly view her as untrustworthy.
As recently as Monday, Clinton was declining to directly apologize for her actions. Asked about a potential apology by The Associated Press, Clinton would only speak vaguely about taking "responsibility."
"Well, I understand why people have questions and I'm trying to answer as many of those in as many different settings as I can. What I did was allowed by the State Department. It was fully above board," she said. "And I've also tried to not only take responsibility, because it was my decision, but to be as transparent as possible."
Clinton also offered something just short of an apology during an MSNBC interview last week.
"At the end of the day I am sorry that this has been confusing to people," she said.