"I had not seen an attack of such ferocity and intensity previously in Libya nor in my time with the Diplomatic Security Service," said the official, Eric A. Nordstrom, who cautioned against overreacting to the episode.

He said that the answer to "a new security reality" should not be "to operate from a bunker."

Mr. Nordstrom presented his testimony at the first hearing held by a Congressional committee investigating the attacks that led to the deaths of Ambassador J. Christopher Stevens and three other Americans.

With less than a month before Election Day, the hearing had highly charged political overtones. Republicans have charged that the Obama administration has played down the significance of the attacks and what they say are the policies that allowed them. Democrats have responded that Republicans are trying to politicize the episode.

In his opening statement, the committee chairman, Representative Darrell Issa, Republican of California, said that "on a bipartisan basis," the committee would try to reassure Americans serving overseas that they were protected. He also praised Secretary of State Hillary Rodman Clinton for cooperating with the committee.

But the committee's ranking Democrat, Representative Elijah Cummings of Maryland, challenged that assertion. In his statement, he said Republicans had withheld documents and had not made witnesses available for interviews. He also called on the House to restore what he said was "hundreds of millions of dollars" it has cut from embassy security financing in recent years.

As a regional security officer, Mr. Nordstrom served in Libya from Sept. 21, 2011, to July 26, 2012, six weeks before the attacks. He said the security plan that was in place at the mission "was regularly tested and appeared to work as planned despite high turnover" of Diplomatic Security agents on the ground.

He said he hoped that in reacting to the events in Benghazi "a new security reality" did not result in overreaction.

"It is critical that we balance the risk mitigation with the needs of our diplomats to do their job, in dangerous and uncertain places. The answer cannot be to operate from a bunker."

Another witness, a National Guard officer who was temporarily deployed in the Tripoli embassy as the site's security officer, said that he became increasingly concerned about what he described as "the weak" security he saw in Benghazi on two visits there last spring and summer.

"The security in Benghazi was a struggle and remained a struggle throughout my time there," said Lt. Col. Andrew Wood of the Utah National Guard, who left Libya in August, some six months after he was deployed to the embassy. "The situation remained uncertain and reports from some Libyans indicated it was getting worse."

The two witnesses presented their testimony at a hearing of the House Committee on Oversight and Government Reform. The committee is investigating the attacks in Benghazi and will focus on security lapses or any potential intelligence failures that resulted in the surprise attacks.

After declining for weeks to provide details about the assault on Sept. 11, the State Department on Tuesday night arranged with little notice a conference call in which a spokesman gave new details on what happened.

The account provided by a State Department official, whom the agency declined to identify, differed from the initial Obama administration reports in some important respects. Susan E. Rice, the American ambassador to the United Nations, had said that the attack on the mission began with an angry protest about an anti-Islamic film that was "hijacked" by extremists.

But the new account provided by the State Department made no mention of a protest. In this account, Mr. Stevens met with a Turkish diplomat during the day of the attack and then escorted him to the main gate of the mission around 8:30 p.m. At that time, there were no demonstrations and the situation appeared calm.

Little more than an hour later, there was gunfire and explosions. American agents, watching the compound through cameras, saw a large group of armed men moving into the Benghazi compound. The barracks for a local militia that was protecting the compound was set on fire, and the attack began to unfold.

Seeking to defend the State Department against charges of lax security, the official suggested to reporters that it could not have been anticipated. "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," he said. "It would be very, very hard to find a precedent for an attack like that in recent diplomatic history."

For all the detail the State Department provided on Tuesday, however, some key questions were not answered. The official declined to say how many security personnel were sent from the embassy in Tripoli to Benghazi on the night of the attack to try to restore order, how many security personnel were based in Tripoli at the time and how long it took the reinforcements to arrive.

On Tuesday, the eve of the hearing, committee members engaged in a series of partisan attacks. Democrats and Republicans said that the other party had shown scant interest in dealing with the broader issues of intelligence warnings and security matters, and had focused instead on trying to show that their party was better equipped to address volatile and shifting national security challenges.

"Never in all of my years in Congress have I seen such a startling and damaging series of partisan abuses," Mr. Cummings said. "The Republicans are in full campaign mode, and it is a shame that they are resorting to such pettiness in what should be a serious and responsible investigation. We should be above that."

Representative Jason Chaffetz, Republican of Utah and chairman of the panel's subcommittee on national security issues, said the Democrats' strategy was to "blame it on politics rather than addressing the nature of the issue."

"They can blame it on politics," Mr. Chaffetz said, "but we are concerned about the more than a hundred embassies and thousands of Americans abroad."