File photo of Hillary Clinton.
A longtime close aide to Hillary Clinton will testify behind closed doors Friday before a controversial congressional committee probing a deadly 2012 attack on a US mission in Libya, the panel said.
Huma Abedin has been a trusted confidante for Clinton, serving with her in the State Department - during which the attack took 2012 deadly attackplace - and continuing as a senior aide on Clinton's 2016 presidential campaign team.
Clinton testifies before the committee in an open session October 22.
"Huma Abedin will testify tomorrow before the Select Committee on Benghazi beginning at 10:00 am (1400 GMT)," committee spokesman Jamal Ware said Thursday, describing it as a "closed, transcribed interview."
The committee is probing the attack of September 11, 2012 in which four Americans were killed in Benghazi, including ambassador Christopher Stevens.
Clinton's Republican critics accuse the Obama administration of not doing enough to provide security for the US mission in the months leading up to the violence, and then of failing to adequately respond to the attack.
Democrats charge that the 17-month-old, Republican-led committee has shifted its focus to the contentious subject of Clinton's use of a private email account and server while she was top diplomat, and that the panel is more interested in ruining Clinton's presidential bid.
The number two Republican in the House of Representatives, Kevin McCarthy, inadvertently suggested last month that such an outcome was an unstated goal of the panel.
"Everybody thought Hillary Clinton was unbeatable, right?" he told Fox News. "But we put together a Benghazi special committee, a select committee. What are her (poll) numbers today? Her numbers are dropping."
McCarthy sought to walk back his comments, but they resonated, and they may have been a factor in his dropping out of this month's race for Speaker of the House.
On Wednesday, another congressional Republican, Richard Hanna of New York, admitted the probe was designed in part to "go after" Clinton.
At the weekend a former investigator with the committee accused Republicans of a biased and "partisan investigation" targeting Clinton.