Struggling to bring an end to a continuing controversy, Hillary Clinton apologized Tuesday for using a private email address when she was secretary of state. In an interview with ABC News, Clinton said she was “sorry” for using a personal email address instead of a government account.  

"That was a mistake,” Clinton told ABC News. “I'm sorry about that. I take responsibility. And I'm trying to be as transparent as I possibly can."

Just one day earlier, the Democratic presidential front-runner had rejected the need to apologize, telling the Associated Press, "What I did was allowed."

Though she apologized, Clinton continued to assert Tuesday there was nothing wrong with using a personal email account while she was at the State Department, from 2009 to 2013.

"What I had done was allowed. It was aboveboard,” she told ABC News. “But in retrospect, as I look back at it now, even though it was allowed, I should have used two accounts. One for personal, one for work-related emails."

Clinton’s campaign has been frustrated for months by a steady drip of details about her email practices while leading the State Department. Clinton did not use a government email and instead used an email address run on a server located in her family’s home in the suburbs of New York City. Concerns about the arrangement have only grown in recent weeks, following reports that the server contained classified information.

Last week, the State Department released 7,000 pages of emails from Clinton, including 150 messages that had been redacted to remove information now deemed classified. On Friday, Clinton sought to dispel questions about the email server in an interview with MSNBC.

“I certainly wish that I had made a different choice, and I know why the American people have questions about it,” Clinton said. Still, she asserted that her private email was “allowed by the State Department,” and that she had not distributed any information considered classified at that time.

When asked why she chose to use an email server in her home, Clinton said that, as she was joining the State Department, she “didn't really stop and think what kind of email system will there be.”

But the Washington Post reported late Friday that Clinton’s family had personally paid a State Department staffer, Bryan Pagliano, to maintain the server during her tenure there. According to the Post, Pagliano did not report the payments on the annual financial disclosures he filed while working at the State Department.

Last week, Pagliano told a congressional committee investigating the deadly attack on the U.S. Embassy in Benghazi, Libya, in 2012, that he would not answer questions about his work on the email server and would, instead, invoke his Fifth Amendment right against self-incrimination.

On Tuesday, the New York Times reported that a special intelligence review, conducted by the CIA and the National Geospatial-Intelligence Agency, found that two emails on her email server contained “top secret” information. The emails had previously been flagged by an inspector general for the intelligence agencies.

The FBI has launched a probe to determine who sent classified information to Clinton’s private email address.

Clinton’s campaign has faced criticism in recent weeks from Democratic allies for not taking the controversy related to her private email seriously. That has prompted a change in tone about the emails -- from joking, to contrite and, now, apologetic.