Sidney Blumenthal has emerged as a key figure in Hillary Clinton's testimony in front of the Benghazi Select Committee, with Republicans honing in on the influence he held with Clinton and the messages the two exchanged while she served as secretary of state.
So who is he?
Blumenthal is a longtime friend to both Bill and Hillary Clinton who came to know them while working as a reporter covering then-Arkansas Gov. Bill Clinton and later the Clinton White House. In 1997 he was hired as a special assistant and adviser to the president and was one of just four witnesses deposed by the Senate when it tried President Clinton on impeachment charges. In 2003 he wrote "The Clinton Wars" about his role in the White House during the Monica Lewinsky controversy.
Blumenthal advised Hillary Clinton's failed 2008 campaign, and Clinton wanted to offer him a job when she became secretary of state. However, the White House blocked the hire and he remained an informal adviser to Clinton.
The release of emails from Clinton's private server has revealed that the two communicated frequently, with Blumenthal frequently passing along new articles, analysis and recommendations on a variety of issues -- including on Libya, where four Americans were killed in 2012.
In her testimony Thursday, Clinton said Blumenthal mostly sent her unsolicited information and advice, some of which she found interesting and others she discarded.
"I think that the sharing of information from an old friend, that I did not take at face value, that I sent on to those who were experts, is something that makes sense," Clinton said. "But it was certainly not in any way a primary source of or the predominant understanding that we had of what was going on in Libya and what we needed to be doing."
Republicans have taken issue with the access Blumenthal, who was not employed by the government, had to Clinton.
This story originally appeared on NBCNews.com