Hillary Clinton continued to insist Friday that she did not knowingly receive or send classified information from her personal email account when she was secretary of state, even as the presidential candidate expressed regret for not using a government email account.

In a rare national TV interview with MSNBCs Andrea Mitchell, Clinton said her position of working with national security matters made her so careful with classified information.

As has been confirmed repeatedly by the inspectors general, over and over, I did not send or receive any material marked classified, Clinton said. We dealt with classified material on a totally different system, including in person and on secure phone lines.

But Clinton admitted that she regretted not using a work email account for State Department matters.

Well, I certainly wish I had made a different choice and I know why the American people have questions about it, Clinton said. And I want to make sure I answer those questions, starting with the fact that my personal email use was fully above board. It was allowed by the State Department, as they have confirmed. But in retrospect, it would have been better -- I take responsibility, I should have had two accounts: one for personal, one for work-related.

Clinton reiterated that she had chosen to use her personal email account as secretary of state because of its convenience.

I had a personal email when I was in the Senate, as the vast majority of senators do, Clinton said. It was very convenient. I did all of my business on personal emails.

Clintons response to a question from Mitchell about her decision-making process, however, may provide fodder for Republicans eager to use the email controversy to weaken the 2016 Democratic presidential frontrunner.

I was not thinking a lot when I got in. There was so much work to be done. We had so many problems around the world -- I didnt really stop and think, What kind of email system will there be? Clinton said.

Clinton received emails on her private server that are now deemed classified. The FBI is investigating who at the State Department sent the now-classified information to Clinton's private email account.

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