A growing controversy over Democrat Hillary Clinton's use of personal email for work while she was United States secretary of state could drag on for months, threatening to cloud the expected launch of her 2016 presidential campaign.

Clinton tried to cool the brewing firestorm late on Wednesday, saying she wanted the State Department to release the emails quickly. But a senior state department official told Reuters on Thursday the task would take time.

‘The review is likely to take several months given the sheer volume of the document set,’ the official said.

At the same time, the department is investigating whether Clinton violated policies intended to protect sensitive information when she conducted all of her official business through a personal account while serving as secretary from 2009 to 2013, the Washington Post reported on Thursday, citing a senior department official.

Using personal email did not automatically break rules, but the department is determining whether work emails sent from that private account contained information that must be handled on a system meeting specific security protocols, the Washington Post reported.

The state department has said there was no prohibition during Clinton's tenure on using personal email for official business as long as it was preserved. It had no immediate comment on the Washington Post report.

Fox News on Thursday released a state department cable sent from the secretary's office in 2011 reviewing some email policies that encouraged employees to avoid conducting official business from personal accounts.

An extended review or investigation could dash any Clinton hopes of putting the controversy to rest quickly.

‘I want the public to see my email,’ Clinton said in a tweet. ‘I asked State to release them. They said they will review them for release as soon as possible.’

The controversy landed Clinton in trouble just as she prepares to enter the race for the Democratic presidential nomination in 2016. It has prompted some Democrats to wonder whether someone else should be their candidate in the bid to succeed President Barack Obama.

A total of 55,000 pages of documents covering the time Clinton was in office has been turned over, according to the State Department. But Clinton and her aides controlled that process, and the emails were not archived on government servers.

Secretary of State John Kerry told reporters during a visit to Saudi Arabia that the State Department would review the documents "as rapidly as possible."

Clinton's tweeted statement came hours after a congressional committee investigating the Sept. 11, 2012, attack on a US diplomatic facility in Benghazi, Libya, subpoenaed her emails.

The US House of Representatives Select Committee on Benghazi demanded all Clinton communications related to the incident and told Internet companies to protect relevant documents.

The panel's Republican chairman, Representative Trey Gowdy of South Carolina, told reporters he wanted the documents within two weeks or a "really good explanation" for why not.

Republicans have scrutinized Clinton's actions regarding the Benghazi attack in which Ambassador Chris Stevens and three others were killed and have also criticized Clinton's transparency and ethics. The former first lady and U.S. senator has been a lightning rod for Republican detractors dating back to the administration of her husband, former President Bill Clinton.

Clinton is the presumptive favorite for the 2016 Democratic presidential nomination, and there was no sign the controversy was forcing a change of plans. A Democratic source familiar with campaign planning said to expect a Clinton announcement on her intentions in the spring.

The Republican National Committee's top lawyer on Thursday asked the State Department's inspector general to investigate if Clinton's email use violated federal law.