Updated 11:15 a.m.
Just before noon on Saturday, Asiana Airlines flight 214 crashed while landing at San Francisco's airport, killing at least two and injuring dozens of the 307 passengers and crew on board.
While it may be sometime before an official conclusion about what went wrong is released, there are already many clues about possible causes for the crash.
In an interview with "Face The Nation," National Transportation Safety Board Chairman Deborah Hersman said an important system that helps planes land had been turned off at the San Francisco airport.
"What we do know is that there was a notice to airmen that indicated that the 'glide slope' was out," Hersman said, adding it had been out since June due to construction at the airport.
CBS News aviation and safety analyst Capt. Chesley Sullenberger told CBS affiliate KCBS in San Francisco the FAA-mandated construction is intended to increase the safety zone of the runway by moving it farther away from the seawall near where the plane crash landed.
A 2005 advisory issued by the FAA says "the goal is to construct an available runway length for new runways or extensions to existing runways that is suitable" for different kinds of airplane operations.
Even without the construction, officials say it is not unusual for the "glide slope" or "glide path" system to be turned off during good weather conditions.
Sullenberger told KCBS the system triggers "automatic warnings that would occur in the cockpit when you deviate below the desired electronic path."
Numerous eyewitnesses said the plane appeared to be coming in low before crashing.
Ki Siadata, a San Francisco resident whose home overlooks the airport, told CBS News correspondent John Blackstone this plane landing looked different.
"All of a sudden as he got right up to the runway, we just heard a really loud boom, and before we knew it, the plane had come to a stop about 50 yards down," Siadata said. "It was lost in a sea of smoke and black dust."
Crash survivor and San Francisco resident Benjamin Levy told reporters he flies into the airport often, and that he felt the plane was "too low, too soon," just before impact.
"He was going down pretty fast, and I think he just realized he was down too fast," Levy said, adding the pilot seemed to push hard on the engines "just as we were about to hit the water."
NTSB Chairman Hersman would not speculate on "Face the Nation" about whether or not the NTSB investigation has found the plane was too low. Hersman said it was possible, but would not blame the lack of a "glide slope" system for the problem. She said there are more "primitive" options than the "glide slope" system for pilots seeking landing assistance, like lights and on board GPS systems.
In a press conference Sunday, Asiana Airlines officials said it is unlikely that a mechanical failure caused the crash. The plane that crashed, a Boeing 777-200, has an excellent air safety record, something that would corroborate their claim.
After apologizing to victims and their families, the airline's president, Yoon Young-doo, defended the pilots of the plane as "skilled" veterans, saying three had logged more than 10,000 hours each of flight time, while the fourth had put in almost that much time. All four pilots are South Koreans.
Crash survivor Levy said "the crew was not expecting it at all," an account that other passengers have corroborated. Levy described the crew as "overwhelmed" by the "chaos" that erupted on board after the tail ripped off, adding that he and other passengers ended up escorting survivors off the plane to safety.
Sullenberger, who famously piloted the "Miracle on the Hudson" plane that safely landed in New York City's Hudson River, told "CBS Evening News" Saturday anchor Jim Axelrod that San Francisco International has unique terrain.
"In fact, the FAA classified it as a special airport, along with other airports worldwide that involve mountainous terrain or other special challenges," he said. "It is surrounded by water, and of course water is a featureless terrain where depth perception can sometimes difficult be. There are shifting winds, low visibilities, so there are several things that make it special, plus high terrain just past it."
NTSB Chairman Hersman said the investigation has already gathered flight voice and data recorders for analysis.
"Everything is on the table right now" for explaining the crash, Hersman said.
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