Injured Flight Attendants Could Help Explain Why Crashed Asiana Airliner Wasn't Evacuated Immediately

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Six of the 12 flight attendants aboard the South Korean flight that crashed on landing last weekend in San Francisco still haven't been interviewed and could help explain why the pilots initially told them not to evacuate the plane, federal investigators said Wednesday.

The doors on Asiana Flight 214 weren't opened until about 90 seconds after the plane had come to a full stop Saturday, Deborah Hersman, chairman of the National Transportation Safety Board, said at a news conference. The standard is to have the plane fully evacuated within 90 seconds, the NTSB said this week.

Hersman said the pilots at first told passengers to remain seated, but they reversed their decision and ordered an evacuation after a flight attendant told them he saw fire and smoke outside the window.

(Read More: NTSM Focuses on Final Minutes Before Plane Crash)

Many other questions remain unanswered. For example, the pilot told investigators that he saw a flash of light at 500 feet as the plane was descending into San Francisco International Airport on Saturday. And it remained unclear why the plane's evacuation chutes prematurely opened inside the cabin on the plane's second impact — they're supposed to open outward.

Most of the flight attendants who haven't been interviewed were stationed in the rear of the plane, which was most heavily damaged, and could have had a better view of what happened when. They haven't been questioned because they're still being treated for injuries.

Three of them were ejected from the flight in their seats, one of them suffering massive head injuries and another a broken leg. Two more were pinned under evacuation slides, one of whom has been interviewed, Hersman said.

The first officer on a United Air Lines 747 that was waiting to take off on the same runway said in a report to his airline that he saw "two survivors who were stumbling but moving."

"I saw one survivor stand up, walk a few feet, then appear to squat down," the officer said. "The other appeared to be a woman and was walking and then fell off to her side and remained on the ground until rescue personnel arrived."Investigators are also looking closely at whether the pilots may have been fatigued, Hersman said.

Both of the main pilots told investigators that they had had plenty of rest, including eight hours of sleep, before the flight took off from Seoul, South Korea, she said. They had an additional five-hour break during the flight, resuming control from a relief crew about 90 minutes before the Boeing 777 clipped the seawall at the airport, shearing off the plane's tail.

(Read More: Korean Culture May Offer Clues to Asiana Crash)

"We do, in our investigations, look at pilots' 72-hour history — their work and rest history — to try to understand if they were fatigued [and] what conditions they were working under," she said.

Hersman also said flight data indicated that the plane's automated systems recorded "multiple autoplilot and autothrottle modes" as it approached San Francisco — and it wasn't immediately clear why. Investigators are trying to determine "what those modes were and if the pilots understood what the mode was doing," she said.

"There is automation there to support the pilots, but pilots also have to fly the airplane," Hersman told NBC News' Tom Costello. "They have to monitor, and they have to fly."

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Hersman said investigators would be done with the runway itself within 24 hours, which will allow work on repairing it to begin.

Meanwhile, 20 of the crash victims remained in San Francisco-area hospitals, four of them in critical condition, and some could face many months of recovery.

One of those critically injured is a child who is being treated at San Francisco General Hospital, the hospital said.

Questions so far have centered on the jet's air speed and alignment as it approached the runway. At least one of the pilots has told investigators that the crew was struggling to line the jet up properly and hadn't noticed that their air speed had fallen.

(Read More: Captain of Crashed San Francisco Plane Was 'In Training')

The pilot, Lee Kang Guk — a veteran behind the stick of other jetliners — was only halfway through the certification training process for the 777. He was being trained by another veteran pilot — was making his first trip as an instructor, the NTSB said.

The two passengers who were killed were both 16-year old Chinese girls who were on their way to summer camp in the U.S. China's consul general in San Francisco has set up a liaison office in the hotel where most of the Chinese survivors of the crash are staying, the Foreign Ministry said.