Huma Abedin, a trusted advisor to Clinton during her State Department years and now a vice chairwoman of her presidential campaign, appeared composed as she entered a closed-door session of the House Committee on Benghazi.
She was expected to spend hours focused on the attacks of September 11, 2012 that killed four Americans, including ambassador to Libya Christopher Stevens.
The panel's chairman, Trey Gowdy, did not attend.
The special investigation faces fresh scrutiny 17 months after it was empanelled, with Democrats charging it is more interested in ruining Clinton's presidential bid than reaching conclusions about how the executive branch handled the attack and its aftermath.
Number two House Republican Kevin McCarthy inadvertently suggested last month that such an outcome was an unstated goal of the panel.
This week another congressional Republican, Richard Hanna, admitted the probe was designed in part to "go after" Clinton, who testifies before the committee next Thursday.
