Sidney Blumenthal, a longtime ally of Hillary Rodham Clinton who was an unofficial adviser when she was U.S. secretary of state, arrived to testify on Tuesday behind closed doors before the House of Representatives panel probing the deadly 2012 attacks in Benghazi, Libya.

Although congressional investigators subpoenaed Blumenthal to require his attendance, he promised to cooperate with the committee. The panel was expected to ask him about memos he emailed to Clinton about Libya around the time of the Sept. 11, 2012, Benghazi attacks that killed U.S. Ambassador Chris Stevens and three other Americans.

The committee's chairman, South Carolina Republican Trey Gowdy, has said many questions remain unanswered about what happened in Benghazi. But Democrats say Republicans are using the committee investigation to try to discredit Clinton's past management of the State Department as she runs for president in 2016.