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SAN FRANCISCO (CBSNews.com)- The pilot at the controls of an Asiana plane that crashed was guiding a Boeing 777 into the San Francisco airport for the first time, andtried but failed to abort the landingafter coming in too slowly to set down safely, aviation and airline officials said Sunday.

It was unclear whether the pilot's inexperience with the aircraft and airport played a role in Saturday's crash. Officials were investigating whether the airport or plane's equipment could have also malfunctioned.

Also Sunday, San Mateo County Coroner Robert Foucrault said he was investigatingwhether one of the two teenage passengers killed Saturday actually survived the crash but was run over by a rescue vehiclerushing to aid victims fleeing the burning aircraft. Remarkably, 305 of 307 passengers survived the crash and more than a third didn't even require hospitalization. Only a small number were critically injured.

Deborah Hersman, head of the National Transportation Safety Board, said the slow speed of Flight 214 in the final approach triggered a warning that the jetliner could stall, and an effort was made to abort the landing but the plane crashed barely a second later.

At a news conference, Hersman disclosed the aircraft was traveling at speeds well below the target landing speed of 137 knots per hour, or 157 mph.

"We're not talking about a few knots," she said.

Hersman described the frantic final seconds of the flight as the pilots struggled to avoid crashing.

Seven seconds before the crash, pilots recognized the need to increase speed, she said, basing her comments on an evaluation of the cockpit voice and flight data recorders that contain hundreds of different types of information on what happened to the plane. Three seconds later, the aircraft's stick shaker -- a piece of safety equipment that warns pilots of an impending stall -- went off. The normal response to a stall warning is to boost speed and Hersman said the throttles were fired and the engines appeared to respond normally.

At 1.5 seconds before impact, there was a call from the crew to abort the landing.

The details confirmed what survivors and other witnesses said they saw: an aircraft that seemed to be flying too slowly just before its tail apparently clipped a seawall at the end of the runway and the nose slammed down.

Pilots normally try to land at the target speed, in this case 137 knots, plus an additional five more knots, said Bob Coffman, an American Airlines captain who has flown 777s. He said the briefing raises an important question: "Why was the plane going so slow?"

The plane's Pratt & Whitney engines were on idle and the pilots were flying under visual flight rules, Hersman said. Under visual flight procedures in the Boeing 777, a wide-body jet, the autopilot would typically have been turned off while the automatic throttle, which regulates speed, would been on until the plane had descended to 500 feet in altitude, Coffman said. At that point, pilots would normally check their airspeed before switching off the auto throttle to continue a "hand fly" approach, he said.

There was no indication in the discussions between the pilots and the air traffic controllers that there were problems with the aircraft.

The airline said Monday in Seoul that the pilot at the controls had little experience flying that type of plane and was landing one for the first time at that airport.

Asiana spokeswoman Lee Hyomin said Lee Gang-guk, who was at the controls, had nearly 10,000 hours flying other planes but only 43 in the 777, a plane she said he still was getting used to flying. Another pilot on the flight, Lee Jeong-min, had about 12,390 hours of flying experience, including 3,220 hours on the 777, according to the Ministry of Land, Infrastructure and Transport in South Korea. Lee was the deputy pilot, tasked with helping Lee Gang-guk get accustomed to the 777, according to Asiana Airlines.

Among the questions investigators are trying to answer was what, if any, role the deactivation of a ground-based landing guidance system due to airport construction played in the crash. Such systems help pilots land, especially at airports like San Francisco where fog can make landing challenging. The conditions Saturday were nearly perfect, with sunny skies and light winds.

The flight originated in Shanghai, China, stopped over in Seoul, South Korea, before making the nearly 11-hour trip to San Francisco. The South Korea-based airline said four South Korean pilots were on board, three of whom were described as "skilled."

© 2013 CBS Interactive Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.

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