House Benghazi panel calls on Hillary Clinton to testify in May hearing
Former Secretary of State Hillary Clinton on Jan. 30, 2014. [Agence France-Presse]

The congressional panel investigating the 2012 attacks on a US mission in Benghazi called on former secretary of state Hillary Clinton on Thursday to testify at two public hearings, the first next month.

Benghazi Commission chairman Trey Gowdy wrote to an attorney for Clinton requesting her participation in a hearing during the week of May 18, with the precise day to be determined later.

Clinton has declared her candidacy for president in 2016, and she would be expected to temporarily come off the campaign trail in order to testify on Capitol Hill.

The initial appearance will focus on Clinton's exclusive use of a private email account and server while running the State Department from 2009 to 2013, a development that came to light in early March.

Clinton says she delivered all her official work-related emails to the State Department in December, while deleting about 30,000 personal emails, a move widely criticized by Republicans.

If the initial hearing over emails "results in assurances the public record is indeed complete," the committee will then schedule Clinton's second public hearing on Benghazi no later than June 18, Gowdy wrote to Clinton lawyer David Kendall.

Four Americans including US ambassador Christopher Stevens were killed in the attacks on US facilities in the east-Libyan city.

Clinton was the top US diplomat at the time, and Republicans have repeatedly accused her of negligence in responding to the attacks and downplaying their terrorist nature.

Clinton already testified to Congress about Benghazi, in January 2013. Democrats see the congressional enquiry by Republicans as a full-court press against the White House frontrunner aimed at derailing her nascent campaign.

Democrats hit back Wednesday amid revelations by Gowdy's team that the committee may not finish its Benghazi investigation until 2016, in the heart of the presidential election cycle.

"There have already been 21 congressional hearings, five independent or bipartisan reports, and millions of tax dollars spent in the process of investigating this three-year-old tragedy," Clinton campaign chairman John Podesta said.

"This investigation would now be longer than the investigations of Iran-Contra, the Kennedy assassination, Watergate and 9/11."