The U.S. diplomatic post in Benghazi, Libya, was operating under a lower security standard than a typical consulate when it was attacked this month, according to State Department officials.

The mission was a rented villa and considered a temporary facility by the agency, which allowed a waiver that permitted fewer guards and security measures than a standard embassy or consulate, according to the officials.

There was talk about constructing a permanent facility, which would require a building that met U.S. security and legal standards, the officials said.

Allowing a waiver would have been a decision made with input from Washington, Libyan officials and the ambassador, according to diplomatic security experts.

"Someone made the decision that the mission in Benghazi was so critical that they waived the standard security requirements, which presents unique challenges to the diplomatic security service as you can imagine," said Fred Burton, vice president for Intelligence at STRATFOR, an intelligence analysis group.

While standards were lower at the compound, security had been enhanced at the post after a number of incidents this year that included a failed bombing attempt against the compound in June, according to sources.

Several security changes were made over the past few months, the officials said. These included additional barriers and barbed wire, increased lighting, chain link fences, additional sand bags and closed circuit television.

Every U.S. building on the compound was also fitted with a safe room with a steel door, although the officials recognize that the room was not fireproof.

Ambassador Christopher Stevens and the other Americans killed in the attack are believed to have died of asphyxiation from heavy smoke.

"We took the place and made improvements to it in a continued fashion," one official said, adding that a pre-September 11 review of base security deemed the post to be adequately secured given the threat.

The threat assessment was based in part on the June attack and previous attacks on other foreign targets, which included the attempted kidnapping of a Red Crescent staff member, a bomb attack on a United Nations convoy, a rocket attack on an International Committee of the Red Cross facility, and a similar attack on the British ambassador's convoy.

The protections in place reflected the State Department's understanding of the threat, which did not suggest a swarming attack by a militia, the officials said.

U.S. officials have said the attack was a spontaneous outgrowth of a protest over an anti-Muslim film. Officials told CNN's Security Clearance last week there were questions about whether there was a significant protest ahead of time.

There was no indication anything was amiss a couple of hours before before the attack was believed to have started, according to one American who was in Benghazi.

The American, who spoke to CNN on Monday on the condition his name not be used, said he had been speaking with Stevens less than three hours before the deadly attack.

He said there was no indication during the conversation that anything was wrong, saying Stevens sounded upbeat and calm and he was very enthusiastic about the next day's meeting with the American.

About 20 minutes after that conversation, the American spoke with a U.S. security officer to discuss logistics about the upcoming meeting. That call also revealed no signs of any pending trouble, he said.

The American had a second call with the security officer a little more than an hour later, around 9:30 p.m. local time, and the situation had dramatically changed. The security officer told the American "we've got a real problem here" and hung up.

The timing matches what CNN's Arwa Damon, who has been reporting from Benghazi, said about the September 11 attack. Damon reported a group of armed individuals showed up outside the consulate around 9:30 and a firefight broke out within 30 minutes.

The American source was at his hotel, located a little more than a mile from the diplomatic post, when he heard large explosions coming from the direction of the mission seconds after the phone call.