WASHINGTON (AP) — Even if Hillary Rodham Clinton's use of a private email server to conduct official business as secretary of state was not illegal, it violated Obama administration guidance and undermined the president's pledge of historic transparency.

Clinton's reliance on hdr22@clintonemail.com for electronic communication as the nation's top diplomat ignited another political controversy for the presumptive 2016 Democratic presidential front-runner, who is no stranger to scandal. It was a puzzling and risky decision for a political figure who is vulnerable to criticism about trying to operate outside the rules.

Clinton has yet to explain why she took to unusual step of using her own server and eschewing a State Department email address. That has left the White House that vowed to run the most open government in history in the awkward position of having to answer for her electronic secrecy.

For the second day in a row Wednesday, Clinton's emails dominated the White House press briefing. Press secretary Josh Earnest carefully avoided direct criticism of Clinton while making it clear that administration policy is for government emails to be used for official business.

"Very specific guidance has been given to agencies all across the government, which is specifically that employees of the Obama administration should use their official email accounts when they're conducting official government business," Earnest said.