Former Secretary of State Hillary Clinton was asked by a congressional investigator more than two years ago if she used a private email account for government business, but she never responded.

Clinton's controversial use of a personal email account and private server in her role as secretary of state is bound to be a serious issue in her bid for the presidency. She revealed publicly for the first time last month that she used a private account to conduct all government business. She has explained it was more convenient for her to do so. But it had the effect of shielding much of her correspondence from government and media oversight. She had no government email address during her entire four years as secretary of state and there were no safeguards to preserve her correspondence as required by the Federal Records Act.

"Have you or any senior agency official ever used a personal email account to conduct official business?" Darrell Issa, the Republican chairman of the House Oversight Committee, wrote to Clinton in a 2012 letter obtained by the New York Times. "If so, please identify the account used." The congressman also asked for the department's written policy concerning the use of personal email for government business. In addition, Issa inquired about the use of text messages and alias email accounts, CNN reports.

Clinton stepped down as secretary of state weeks later without responding to Issa. Department officials later provided Issa a brief description of the policy on the use of personal emails. According to a letter from State Department legislative affairs official Thomas Gibbons at the time, anyone using a personal account "should make it clear that his or her personal email is not being used for official business." Gibbons did not say whether Clinton had used a private account.

Issa's letter was part of an investigation into suspected use of personal emails for government businesses by some members of the Obama administration and the Environmental Protection Agency.

A Clinton aide said in a statement to the Times that her use of a private email account for government business was "widely known to the over 100 department and US government colleagues she emailed, as her address was visible on every email she sent."