The US government is set to examine thousands of Hillary Clinton's emails for public release - and for possible security lapses - after revelations she used a private account to conduct official business as secretary of state, a senior official said.
Ms Clinton's extensive use of private emails has raised questions in the build-up to her expected presidential run about whether she adhered to the letter or spirit of accountability laws.
The official said the department would review 55,000 pages of emails amassed from Ms Clinton's personal files to determine if there were any instances where she improperly transmitted sensitive information.
US state department policy holds that information that is not classified as secret but contains sensitive national security or diplomatic information can only be conveyed on secure channels except for certain circumstances, the official explained.
Ms Clinton's extensive use of private emails heightened security risks for her communications, chiefly the potential for inadvertent disclosure of such sensitive information and danger from hackers, several information security experts have said.
US secretary of state John Kerry, currently in Saudi Arabia, said his department "will undertake this task as rapidly as possible in order to make sure that we are dealing with the sheer volume in a responsible way".
Officials said the review could take months to complete, potentially a drawn-out distraction for Ms Clinton and an unnerving development for the many Democrats who see her as the party's presidential nominee-in-waiting for 2016.
The review was prompted by the disclosure that Ms Clinton, in a departure from predecessors, relied exclusively on a private account for emails about government business. The emails were sent from a private computer server using an internet address that traces back to Clinton's family home in Chappaqua, New York.
The department announced the review soon after Ms Clinton addressed the matter for the first time, saying on Twitter: "I want the public to see my email. I asked State to release them."
Clinton aides and the State Department both say she never received or transmitted classified information on her private email account. But unclassified diplomatic details and internal matters are sometimes considered sensitive and can be targeted by hackers and foreign governments.
"She had other ways of communicating through classified email through her assistants or her staff," said Marie Harf, speaking for the state department. Officials have said Ms Clinton turned over more than 55,000 pages of emails to the department.
The former first lady's use of a private email account for official business appeared to contradict instructions from her own office in June 2011 to all State Department employees.
In a cable from Ms Clinton's office, employees were advised to "avoid conducting official Department business from your personal e-mail accounts".
It cited reports that unspecified "online adversaries" were targeting personal email accounts of state department employees. The cable was first obtained by Fox News.
The state department said Ms Clinton's instructions in 2011 only applied to emails containing "sensitive but unclassified" information, a category that includes personal information about employees or the public, business secrets, details of ongoing investigations or records about visa or asylum applications.
Only one instruction in the cable directly referred to rules about such information, an existing ban on anyone auto-forwarding government emails to their personal accounts.
Even though the instructions bore Ms Clinton's name, it was not clear whether she wrote them or was aware of them. Virtually every cable leaving the department's headquarters in Washington would have had Ms Clinton's name on it while she was in town.
Ms Clinton's private email practices gave her significant control over access to her message archives, highly unusual in government. They also could complicate the state department's legal responsibilities in finding and turning over official emails in response to any investigations, lawsuits or public records requests.
The matter also raises questions about whether anyone in government examined Ms Clinton's private email server and network before it began operating and continued to review it regularly during her tenure.
Federal regulations subject the computer systems of some contractors and other organizations to federal oversight when they interact with government systems to ensure they are protected.
Under the Freedom of Information Act, the US government can censor or withhold emails to protect information that would hurt national security, violate personal privacy or expose business secrets or confidential decision-making in certain areas.
It was not clear whether the state department would automatically apply those provisions to its review of Ms Clinton's emails, or use its discretion to release even emails that might be covered under those exemptions.
Withholding emails merely because they might be embarrassing or expose government incompetence or malfeasance is not permitted under the act's guidelines.
The state department is already overwhelmed with nearly 11,000 pending requests for various emails under the open records law.
More than 75 separate requests for Clinton material were filed with the State Department between 2009 and 2013 by media organisations and others. Some requests for Clinton emails and other documents have been delayed for more than a year - and in one case, four years - without any results.
On Wednesday, the House committee investigating the attacks on the US diplomatic compound in Benghazi, Libya, issued subpoenas for emails from Ms Clinton. It also instructed technology companies it did not identify to preserve any relevant documents in their possession.
The White House legal counsel's office was not aware of Ms Clinton's private email account until the committee sought her communications during an earlier exchange with the department, according to a person familiar with the matter.