US Secretary of State Hillary Clinton is expected to testify to Congress over the deadly attack on a US mission in Benghazi, Libya, last year.
Mrs Clinton will face questions about security failures that led to the attack before the foreign relations committees of the Senate and the House.
The US envoy to Libya Christopher Stevens and three other officials were killed in the attack on 11 September.
The ambassador died of smoke inhalation when he was trapped in the burning consulate building, after armed men had stormed the compound.
The assault triggered a major political row over who knew what and when. As a result, an independent panel - the Accountability Review Board- was charged with investigating the incident.
Democrats hold the majority in the Senate, where Mrs Clinton will be surrounded by former colleagues and the tone will likely be respectful, says the BBC's Washington correspondent, Kim Ghattas.
In the House, however, Mrs Clinton is expected to face much more heat.
The panel review did not blame her directly for any of the failures, but members of Congress will still want to know why she was not personally aware of requests for more security in a high-risk posting like Libya, our correspondent adds.
She will also face questions about how the administration of President Barack Obama handled the fallout.
Three State Department employees have been fired over the Benghazi attack, and recommendations the panel made in December are already being implemented.
Mrs Clinton, who is stepping down from her post in two weeks, spent a month recuperating from a series of ailments in December.
These include a concussion after she became dehydrated as a result of a stomach virus and a blood clot near her brain.
Mr Obama appointed Mrs Clinton at the start of his first term in 2009. She is considered a strong candidate for the Democratic nomination for president should she run in 2016.
Mr Obama has nominated Democrat Senator John Kerry - who is expected to be swiftly confirmed - as her replacement.
Mrs Clinton is scheduled to testify for 90 minutes before the committees.