Hillary Clinton used a personal email account for official business during her tenure in the U.S. Senate and carried the practice over once she was at the helm of the State Department, an aide to the presumptive Democratic nominee for president said in sworn testimony released Tuesday.

Cheryl Mills, Mrs. Clinton’s chief of staff during the White House hopeful’s four-year stint with the State Department, said her former boss relied on a personal email account provided by AT&T for about three months after being sworn in as secretary of state by President Obama in January 2009.

“Secretary Clinton continued a practice that she was using of [sic] her personal email,” Ms. Mills testified Friday, according to a transcript of her remarks released this week by Judicial Watch, a conservative watchdog group currently suing the State Dept. over Mrs. Clinton’s use of a nongovernmental email system while in office.

Ms. Mills’ admission comes amid an active FBI probe launched to investigate Mrs. Clinton’s use of a private email account while secretary of state, in addition to the Judicial Watch lawsuit. Leaked emails released to the media in 2013 first indicated that Mrs. Clinton used an account hosted at “clintonemail.com” in lieu of a government-provided account, raising questions regarding security concerns and, as argued by some, possible efforts to conceal correspondence from Freedom of Information Act requests.

As revealed through Friday’s sworn testimony, however, Mrs. Clinton relied on a separate, nongovernmental account through as late as April 2009 before she abandoned her AT&T account and began communicating through the clintonemail.com account.

“So Secretary Clinton used — always used one email account when she was using an email account,” Ms. Mills testified. “So when she initially arrived [at the State Dept.] she was continuing to use the AT&T accounts, and then transitioned to the .Clinton email, or Clintonemail.com account. And during her tenure those were the two addresses, if you will, that she used.”

“I don’t know that I could articulate that there was a specific discussion as opposed to her continuation of the practice she had been using when she was a senator. … I don’t have a specific memory of the conversations that may or may not have occurred. I know that I understood she was going to be using her personal email and that’s what she did,” Ms. Mills testified.

Christopher Soghoian, the principal technologist at the American Civil Liberties Union, tweeted Wednesday that Mrs. Clinton’s was likely not the only lawmaker on Capitol Hill to use personal email accounts for official business, but said it nevertheless poses “a huge cybersecurity problem.”

Rep. Jason Chaffetz, Utah Republican, acknowledged last year that his personal Gmail address is listed on his congressional business card in lieu of a government-provided email account, but explained to ABC News at the time that members of Congress are not subject to the Federal Records Act.

Mr. Chaffetz, who chairs the House Oversight and Government Reform, has overseen the federal investigation concerning the 2011 terrorist attack in Benghazi, and he has been among the most vocal critics in Congress with respect to Mrs. Clinton’s use of a personal email account in communicating official business in the immediate aftermath of the assault.

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