- Top State Department officials will appear before the House Oversight Committee
- The Sept. 11 attack killed the U.S. ambassador to Libya and three other Americans
- Republicans argue President Obama's policies caused vulnerability to such an attack
- The administration changed description of event from a protest gone awry to a terror attack
Washington (CNN) -- A congressional hearing on Wednesday loaded with political implications will examine the terrorist attack in Libya that killed U.S. Ambassador Christopher Stevens and three other Americans on the anniversary of 9/11.
The Republican-led House Oversight Committee scheduled the hearing even though Congress is on leave until after next month's election.
GOP challenger Mitt Romney has made the Libya attack a focus of his criticism of President Barack Obama's foreign policy.
With polls showing more people favor Obama over Romney on foreign policy, the former Massachusetts governor seeks to gain ground by arguing the president has made America less influential and more vulnerable around the world.
The assault in Benghazi, Libya, occurred 11 years to the day after the September 11, 2001, attacks on New York's World Trade Center and the Pentagon. After initially blaming the violence on a protest over an anti-Islam film produced in America, the Obama administration conceded it was a terrorist attack.
At Wednesday's hearing, Under Secretary of State for Management Patrick Kennedy will provide the first direct rebuttal of allegations by Oversight Committee Chairman Darrell Issa, R-California, and others that the State Department denied requests for additional security in Libya.
Others scheduled to testify include Deputy Assistant Secretary of State for International Programs Charlene Lamb; Regional Security Officer Eric Nordstrom, who was stationed in Libya before the attacks; and Lt. Col. Andrew Wood, a Utah National Guardsman who was leading a security team in Libya until August.
Issa's committee had asked Secretary of State Hillary Clinton to take part, and she sent Kennedy and Lamb to appear.
Democrats accuse Issa of planning a partisan hearing, a similar allegation leveled against the panel for its past investigations of the botched "Fast and Furious" gun-running program and the failed Solyndra clean energy company that received about $500 million in government loan guarantees.
On Tuesday, two senior State Department officials provided reporters with the most detailed explanation yet of the attack in Benghazi, telling a conference call that there was no prior indication such an assault was imminent.
The officials, who briefed reporters on condition of not being identified by name, said there was "nothing unusual" throughout the day of the attack.
Stevens held an evening meeting with a Turkish diplomat and then retired to his room in one of the compound's buildings at 9 p.m., according to the officials. The first sign of a problem came 40 minutes later when diplomatic security agents heard loud talking outside the compound, along with gunfire and explosions.
Asked whether the attack was a spontaneous assault taking advantage of a demonstration, as originally asserted by Obama administration officials, one senior official said, "That was not our conclusion."
The two senior officials offered riveting detail of the attack by what one of them described as "dozens of armed men" who marauded from building to building in the enormous complex and later fired mortars on a U.S. annex less than a mile away.
In the havoc at the compound, which had four buildings, Stevens and two of his security personnel took refuge in a fortified room that the attackers were able to penetrate, one official said.
The attackers doused the building with diesel fuel and set it ablaze, and the three men decided to leave the safe haven and move to a bathroom to be able to breathe, according to the official. Stevens became separated from the security personnel in the chaos and smoke, and eventually turned up at a Benghazi hospital, where he was declared dead.
Hospital personnel found his cell phone in his pocket and began calling numbers, which is how U.S. officials learned where he was, the State Department officials said.
The officials echoed what administration officials have maintained since the attack: that U.S. and Libyan security personnel in Benghazi were outmanned and that no reasonable security presence could have fended off the assault.
"The lethality and the number of armed people is unprecedented," one official said. "There had been no attacks like that anywhere in Libya -- Tripoli, Benghazi or anywhere -- in the time that we had been there. And so it is unprecedented, in fact, it would be very, very hard to find precedent for an attack like (it) in recent diplomatic history."
CNN's Jill Dougherty, Elise Labott and Tom Cohen contributed to this report.
