Hillary Clinton apologized for her use of a private email server when she was U.S. secretary of state in an effort to curtail the controversy that continues to weigh down her presidential campaign.

The 2016 hopeful has faced ongoing criticism since it was discovered that she may have stored classified material in a private email server while she was working under the Obama administration. Critics say this could have put national security secrets at risk. Her use of the server, which was maintained by a small Denver company and stored at a secure data center in New Jersey, has also prompted an FBI counterintelligence investigation.

During an interview with ABC News' David Muir, which aired on Tuesday, the former first lady said that using a private "homebrew" email server was a "mistake." She also acknowledged that she should have used different accounts to separate her work and personal business.

"That was a mistake. I'm sorry about that. I take responsibility," she said.

Her apology came just one day after she told the Associated Press that she wouldn't apologize for using a personal email address because "what I did was allowed. It was allowed by the State Department. The State Department has confirmed that."

While speaking with Muir, Clinton reiterated that her use of personal email "was allowed" and that "everyone in the government I communicated with knew I was using personal email but I am sorry that it has raised all of these questions." Nonetheless, the former U.S. senator took responsibility "for having made what was not the best decision."

Later that night, the Democratic presidential front-runner wrote an apology in a Facebook post.

"Yes, I should have used two email addresses, one for personal matters and one for my work at the State Department. Not doing so was a mistake. I'm sorry about it, and I take full responsibility," she wrote on Tuesday.

She added, "I know this is a complex story. I could have -- and should have -- done a better job answering questions earlier. I'm grateful for your support, and I'm not taking anything for granted."

Since the controversy over Clinton's emails first emerged in March, Clinton was forced to turn almost half of her emails from her account while deleting an additional 32,000 personal messages. However, at least 305 emails have been flagged for further review by the intelligence community, reports The Guardian.