WASHINGTON — An effective response by newly trained Libyan security guards to a small bombing outside the American diplomatic mission in Benghazi in June may have led United States officials to underestimate the security threat to personnel there, according to counterterrorism and State Department officials, even as threat warnings grew in the weeks before the recent attack that killed Ambassador J. Christopher Stevens and three other Americans.

The guards’ aggressive action in June came after the mission’s defenses and training were strengthened at the recommendation of a small team of Special Forces soldiers who augmented the mission’s security force for several weeks in April while assessing the compound’s vulnerabilities, American officials said.

“That the local security did so well back in June probably gave us a false sense of security,” said one American official who has served in Libya, and who spoke on condition of anonymity because the F.B.I. is investigating the attack. “We may have fooled ourselves.”

The presence of the Special Forces team and the conclusions reached about the role of the Libyan guards offer new insight into the kind of security concerns that American officials had before the attack on Sept. 11.

Security at the mission has become a major issue as the Obama administration struggles to explain what happened during the attack, who was responsible and how the ambassador ended up alone.

Republicans and Democrats in recent days have demanded more detailed explanations from the White House and State Department on possible security lapses. “There were warnings,” Senator John McCain, Republican of Arizona, said on CNN’s “State of the Union” program on Sunday.

Just how much American and Libyan officials misread the threat has become even more evident as they analyze the skill with which the mortar attack at an annex a half mile away was carried out by the attackers. That assault, nearly three hours after the initial attack on the main diplomatic mission, killed two former Navy SEALs who were defending the compound.

With as few as four armed Americans and three armed Libyans guarding the mission as the attack began, Mr. Stevens’s own bodyguard was so far away that he needed to sprint across the compound under gunfire to reach the building where the ambassador was working at the time. But the bodyguard ultimately left without Mr. Stevens, who died of smoke inhalation.

And even after eight additional American security officers arrived from Tripoli, the roughly 30 Americans were surprised and outgunned again in the second attack, dependent on an ad hoc collection of Libyan militiamen to protect their retreat and avoid greater casualties, Libyan officials said.

American counterterrorism officials and Libyans on the scene say the mortar attack was most likely carried out by the same group of assailants who had attacked the mission and then followed the convoy of American survivors retreating to what they thought was a safe house.

The first mortar shell fell short, but the next two hit their mark in rapid succession with deadly precision, according to an account that David Ubben, one of Mr. Stevens’s security guards, told his father, Rex Ubben, which was supported by other American and Libyan officials.

“There are three villas inside and the walls are high, and the only house that got hit was the house we were in,” said Fathi el-Obeidi, a Libyan militia commander who came to help evacuate the Americans.

This indicated that many of the assailants were practiced at aiming their mortars, skills they learned in fighting Col. Muammar el-Qaddafi’s army.

“David did not draw a distinction between the attackers,” Rex Ubben said in a telephone interview. David Ubben, a 31-year old Iraq war veteran, was wounded in the mortar attack, and is recovering from his wounds at Walter Reed National Military Medical Center in Bethesda, Md. His father said he had declined to speak to reporters.

The Sept. 11 attack culminated several weeks of growing violence against Western and other diplomatic posts in Benghazi. State Department officials said they were aware of the worsening climate and took precautions. One American official who worked in the mission said the Americans there were able to get around with “appropriate prudence.”

One American official, who said he traded e-mails with Mr. Stevens three days before his death, said the ambassador did not mention any heightened security concerns. CNN, however, has reported that Mr. Stevens did express such worries in a diary that one of the network’s correspondents found at the ransacked mission.

But security had been a concern for months. After an attack in early April on the convoy of the United Nations special envoy for Libya, Ian Martin, the United States Embassy in Tripoli sent about four Special Forces soldiers to Benghazi to augment security and conduct the security assessment, the American official said. The soldiers were part of a larger group of nearly two dozen Special Operations personnel, including Navy SEALs and bomb-squad specialists, that the military’s Africa Command sent to Tripoli last fall to establish security at the embassy there.

As a result of the military assessment, the mission increased the number of sandbagged defensive positions and gave the Libyan security guards more training. “We weren’t blind to fact the security situation in Benghazi was more tenuous than in Tripoli,” said the American official who served in Libya. “We were constantly considering Benghazi and constantly looking for ways to improve security there.”

The first test of the new defenses came when militants attacked the mission with a homemade bomb on June 6, the day after the United States announced that it had killed Abu Yahya al-Libi, a top leader of Al Qaeda, in Pakistan. No one was injured in the June 6 bombing.

Representative Peter King, a New York Republican who heads the House Homeland Security Committee, said after the roadside bombing in June, he heard nothing from the State Department or others in the government about a need for more security in Benghazi.

“Between June 6 and Sept. 11, I’m not aware that they asked for more security or that they thought they needed more because it was more of a risk, or that there was talk or a debate about it,” he said.

While the broad outlines of what happened that night have been reported, details continue to emerge that paint a more complete picture of the frantic response to the attack. It began about 9:30 p.m., roughly 15 minutes after Mr. Stevens had finished an evening meeting with the Turkish ambassador, bid him farewell and chatted briefly with a handful of Libyan guards at the gate of the compound.

There were a total of seven Libyan guards at the edge of compound. Four were unarmed guards who worked for the British security firm Blue Mountain inside the gates, checking visitors’ identification, operating a metal detector and running their bags through an X-ray machine. Three others were armed members of a major local militia that fought in the uprising against Colonel Qaddafi, the February 17 Brigade. The brigade had been responsible for securing the mission from its inception, and in interviews the guards said that they had received additional training for the job of guarding the mission.

There were no more than seven Americans in the compound, including three civilians and four who carried guns, three of the Libyan guards later recalled, speaking on condition of anonymity for their safety. In addition to Mr. Stevens, the Libyans said, the civilians included a familiar figure they identified as “the bald maintenance guy” — Sean Smith, a computer technology specialist, as well as another official visiting from Tripoli whom the Libyans referred to as a “delegate.” The Libyan guards said they believed that Mr. Stevens was alone in the residence at the time of the attack, and the locations of Mr. Smith and the visitor at the time were unclear.

Just before 9:30, the Libyan guards began hearing shouts of “God is great” from outside the walls. They said that they had initially assumed the shouts were from a funeral procession.

An unarmed Blue Mountain guard said he tried to call his superior on his two-way radio and could not reach him. Then he heard American voices through the radio: “Attack, attack!”

Moments later the guards heard gunfire, the blasts of rocket-propelled grenade launchers, and other grenades falling inside the compound. The attackers moved on all three entrances at once in an apparently coordinated assault, backed by truck-mounted artillery.

Mohamed Bishari, 20, the son of the landlord and a neighbor who watched the attack, said: “They thought that there would be more Americans inside, commandos or something like that. So they immediately started attacking with their R.P.G. rockets.”

He and other witnesses identified the attackers as Ansar al-Shariah, a well-known brigade of local Islamist militants. He said they arrived waving the black flag favored by such ultraconservative jihadis.

The unarmed Libyan guards ran back to take up positions as they had been instructed, behind sandbags that had been erected between the office and the residence. “The shooting was coming from all directions,” one guard said. “I hid behind the sandbags saying my last prayers.”

Another grenade landed inside the structure housing the three armed Libyan guards but, miraculously, did not explode.

“When the grenade didn’t explode, they came out of the windows,” said one of the unarmed guards, who said he had spoken to the armed contingent over the two-way radio during the attack. “They had a ladder outside the villa which they used to go up on the roof and started resisting.”

“They were resisting and radioing for backup from their brigade at the same time,” the guard said. “They managed to get a few.” Another guard said, “It was like a fog of war, it was chaotic, you couldn’t see anything” He added: “By the end it was every man for himself.”

Three guards, speaking independently, said they saw one of Mr. Stevens’s bodyguards run out of an office building with a light weapon drawn, racing back to the residence under fire to try to protect the ambassador.

Two other security guards, whom the Libyans identified only as Scott and Dave, were in the compound’s canteen and went to its roof to fight, the Libyans said.

Mr. Smith, the information technology worker, died of smoke inhalation during the fight. The American security detail, including Mr. Ubben, was unable to locate Mr. Stevens in the residence because of the thick, choking smoke in the building, and managed only to retrieve Mr. Smith’s body, an American official said.

Previous American government accounts indicated that a convoy evacuated about 20 Americans from the mission at about 11:30 p.m. But Mr. Bishari, the neighbor, said that more than two and a half hours after the fight began, between midnight and 1 a.m., he saw what he described as the ambassador’s armored Mercedes S.U.V. leaving the mission. He pointed to a hole in the compound’s concrete wall that he said was left by a rocket-propelled grenade that was fired at the fleeing vehicle and evidently missed.

The annex building was a secret. The Libyan militia leaders who escorted the Americans say they were unaware of it, and the eight American security officers who arrived at the Benghazi airport from Tripoli at about 1:30 a.m. guided the Libyans to it using a GPS device, members of the Libyan team said.

Those eight Americans initially planned to leave the airport with Mr. Fathi and a handful of Libyan militiamen in four vehicles, two Toyota Land Cruisers followed by two Kia sedans. But when they learned of the Americans’ arrival, local Libyan security forces insisted on sending 16 more vehicles of fighters, Mr. Obeidi said. “I told them not to be too close to us so when we get to the place we don’t create a scene,” he said.

But the attackers had evidently found it, perhaps by following the vehicle leaving the compound. Libyan witnesses who saw the attacks in both locations said they appeared to be the same group, Ansar al-Shariah.

The attackers evidently had set up mortar rounds in advance of the attack. They hit the annex just after the Libyan escort and American security team had reached the gate, Mr. Obeidi said.

United States government officials say they learned from the bodyguards as early as 2 a.m. that Mr. Stevens had disappeared in the smoke. Mr. Obeidi said by that time, he had learned from the hospital that the doctor there who had treated the ambassador identified his body. But other Libyan officials say they were unsure of Mr. Stevens’s condition.

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Correction: October 8, 2012

September 30, 2012
By ERIC SCHMITT, DAVID D. KIRKPATRICK and SULIMAN ALI ZWAY

Find this article at 8 October 2012

© 2012 The New York Times Company