The fatal attack on a U.S. diplomatic compound in Libya last month has become a test of Secretary of State Hillary Rodham Clinton’s leadership and a threat to her much-admired legacy as America’s top diplomat just a few months before she plans to step down.

Clinton was among the first Obama administration officials to publicly condemn the attack and mourn the deaths of Ambassador J. Christopher Stevens and three other Americans. But as the State Department has weathered Republican-led criticism that it misread warning signs before the Sept. 11 attack, Clinton has been far less visible.

Clinton will not appear at a Wednesday oversight hearing on the Libya attack, where House Republicans have said they will question the State Department’s security preparations and the administration’s account of the attack. The State Department will instead send a trusted career diplomat along with three security officials.

Ahead of the hearing, State Department officials provided new details about the attack while asserting that there had been no way to predict or prevent the sustained assault.

“The lethality and the number of armed people is unprecedented. There had been no attack like that anywhere in Libya — Tripoli, Benghazi or elsewhere — in the time we had been there,” said one official who spoke on the condition of anonymity to discuss events still being investigated. “It would be very, very hard to find a precedent for an attack like that in recent diplomatic history.”

Documents: Security incidents in Libya since June 2011

But the new details also appeared to confirm that there was no protest or other benign gathering outside the compound gates, as initially described by some in the administration.

A month before the presidential election, with Mitt Romney surging in the polls, Republicans increasingly see the Libya attack as a political vulnerability for President Obama. Republicans accuse Obama and top administration officials of overlooking warning signs before the attack and trying to deflect questions about terrorism afterward.

“There was a clear disconnect between what security officials on the ground felt they needed and what officials in Washington would approve,” Rep. Darrell Issa (R-Calif.), chairman of the House Oversight and Government Reform Committee, said Tuesday. “Reports that senior State Department officials told security personnel in Libya to not even make certain security requests are especially troubling.”

Clinton briefed members of Congress privately days after the attack and has called individual members frequently. She spoke to Issa on Monday, another senior State Department official said.

“They did not ask, and there was no serious discussion at this point that she would testify,” the official said.

Clinton has made no public mention of the attack or investigation since Oct. 3. She has no public speaking events on her schedule this week. A trusted Clinton confidant who is the chief protector of her image is reviewing all media inquiries related to the attack.

“It did happen on her watch, so is the secretary responsible? The secretary is always responsible,” said P.J. Crowley, a former assistant secretary of state for public affairs under Clinton and now a professor at George Washington University.

“You’ve gotta look at this in the full picture. It’s a tragedy that happened on her watch, but I don’t think it will diminish what is a very significant record,” Crowley said.

Frequently touted as a Democratic presidential candidate in 2016, Clinton plans to step down this year after a tenure that has made her among the most-traveled and most respected American diplomats. Her approval ratings in national polls hover around 70  percent, making her more popular than Obama.

But the deaths in Benghazi have opened Clinton up to the charge that her department should have done more to safeguard diplomats from militants in an increasingly violent country.

Briefing reporters on Tuesday, State Department officials said that Stevens’s trip had been routine before the attack and that on the night of the attack he escorted his last guest out the front gate about 8:30 p.m. The ambassador stood on the quiet street to say goodbye, then retired to his room.

At 9:40 p.m., there was an explosion and gunfire at the gates, and a wave of armed men flowed into the compound. The armed crowd quickly assaulted all four buildings on the compound with mortars, small arms and possibly with rocket-propelled grenades, the official said.

With the building on fire and rapidly filling with smoke, a security agent tried to lead Stevens and information officer Sean Smith out a bedroom window, but when the agent tumbled out, the two others did not follow, the official said. Repeated attempts by the agent to find Stevens and Smith failed.

Neither Romney nor congressional Republicans have directly faulted Clinton over security in Libya and do not appear eager to saddle her with the blame. Instead, much of the Republican criticism has been directed at Susan E. Rice, the U.S. ambassador to the United Nations, who initially said that the attacks were apparently the result of anti-American protests that spun out of control.

Still, Issa sent a stern letter to Clinton last week, asking why additional security had been denied to diplomats at the lightly defended mission in Benghazi where Stevens died. The letter questioned the administration’s early public claim that the attack was part of a spontaneous public protest over an anti-Muslim Internet video.

The State Department has said that a review panel, led by retired diplomat Thomas Pickering, is working to answer such questions.

“Our posture is to be as cooperative as we possibly can,” Victoria Nuland, a State Department spokeswoman, said Tuesday.

Her comments came as the Republican-led oversight committee released the State Department’s compilation of more than 200 security-related threats in Libya since June 2011.

The documents show that less than two months before the attack in Benghazi, the State Department assessed that the risk of violence to diplomats and other Americans in Libya was high and that the weak U.S.-backed government in Tripoli could do little about it.

The department, the documents show, approved a 30 percent “danger pay” bonus for Americans working in Libya during the summer.

The department’s former top security officer has told the House committee that he had recommended keeping U.S. military and additional State Department security forces on hand through October. “The [Libyan government] was overwhelmed and could not guarantee our protection,” former regional security officer Eric A. Nordstrom wrote Oct. 1. “Sadly, that point was reaffirmed Sept. 11, 2012, in Benghazi.”

Nordstrom is scheduled to testify at Wednesday’s oversight hearing.

Although Clinton will not appear, the State Department requested that Undersecretary for Management Patrick Kennedy be a witness. Little known outside Washington, Kennedy is a low-key bureaucratic firefighter who was already managing the back and forth with Congress over the attacks.

Julie Tate in Washington and Michael Birnbaum in Tripoli contributed to this report.