U.S. Secretary of State Hillary Clinton faces tough questions in her long-awaited congressional testimony concerning the assault on the U.S. diplomatic mission in Benghazi, Libya, that killed Ambassador Chris Stevens and three other Americans, The Associated Press reports.

Clinton is the sole witness Wednesday at back-to-back hearings before the Senate and House foreign policy panels on the September raid, an independent panel's review that harshly criticized the State Department and the steps the Obama administration is taking to beef up security at U.S. facilities worldwide.

Clinton had been scheduled to testify before Congress last month, but an illness, a concussion and a blood clot near her brain forced her to postpone her appearance.

Her marathon day on Capitol Hill will probably be her last in Congress before she steps down as secretary of state. President Barack Obama has nominated Sen. John Kerry, D-Mass., to succeed her, and his swift Senate confirmation is widely expected. Kerry's confirmation hearing is scheduled for Thursday, the AP reports.

Clinton's testimony will focus on the attack after more than three months of Republican charges that the Obama administration ignored signs of a deteriorating security situation in Libya and cast an act of terrorism as mere protests over an anti-Muslim video in the heat of a presidential election. Washington officials suspect that militants linked to al-Qaida carried out the attack.