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.