National Transportation Safety Board chairwoman Deborah Hersman said the lower portion of the plane's tail cone was found in rocks inside the seawall. A "significant piece" of the tail of the aircraft was in the water, and other plane parts were visible at low tide, she said.
Hersman said at a news conference that investigators have reviewed airport surveillance video to determine whether an emergency vehicle ran over one of two teenage girls killed in Saturday's crash but have not been able to reach any conclusions.
She called the possibility a "very serious issue."
"I can tell you that the two fatalities were located in seats towards the rear of the aircraft. This is an area of the aircraft that was structurally significantly damaged. It's an area where we're seeing a lot of the critical or serious injuries," Hersman said of the girls' location.
Investigators want to make sure they have all the facts before reaching any conclusions, Hersman said, adding that the coroner has not yet determined the girl's cause of death and is charged with doing so.
San Mateo County Coroner Robert Foucrault earlier said his office was conducting an autopsy to determine whether one of the victims survived the crash but was run over and killed by a responding vehicle. He said his staff was notified of the possibility by senior San Francisco Fire Department officials at the crash site on Saturday.
San Francisco Fire Chief Joanne Hayes-White and Assistant Deputy Chief Dale Carnes both said earlier Monday that one of the two teenage girls killed in the crash might have been struck.
"There was a possibility one of two fatalities might have been contacted by one of our apparatus at one point during the incident," Carnes said.
More than 180 people went to hospitals with injuries. But remarkably, 305 of 307 passengers and crew survived the crash and more than a third didn't even require hospitalization. Only a small number were critically injured.
Investigators said Asiana Airlines Flight 214 was traveling "significantly below" the target speed during its approach and that the crew tried to abort the landing just before it smashed onto the runway. What they don't yet know is whether the pilot's inexperience with the Boeing 777 and at San Francisco's airport played a role. Officials said the probe will also focus on whether the airport or plane's equipment also could have malfunctioned.
One of the issues NTSB investigators are certain to give a hard look is what role pilot fatigue played in the accident. The accident occurred after a 10-hour nighttime flight. As is typical for long flights, there were four pilots on board precisely so that they can switch off in teams of two to get rest. But pilots who are regularly fly long routes say it's very difficult to get restful sleep on planes.
The accident occurred in the late morning in San Francisco, but back in Seoul it was 3:37 a.m. That means that pilots were trying to stay alert and make decisions most likely during their bodies' circadian low — the time of day when people most crave sleep even if they are rested. Studies show that during circadian lows — usually between 3 a.m. and 5 a.m. or 4 a.m. and 6 a.m., depending upon the study — people are often slow to respond and their judgment can be as eroded as much as if they were drunk.
"Fatigue is there. It is a factor," said Kevin Hiatt, a former Delta Air Lines chief international pilot. "At the end of a 10-hour flight, regardless of whether you have had a two-hour nap or not, it has been a long flight."
Hiatt said that when he used to fly international flights "sometimes you got a real quality nap, and other times you had some real noisy passengers. Or you got too hot or too cold, just like a normal passenger. You just didn't get what you thought you should get."
NTSB will also want to know how much rest the pilots had before the flight.
"This one of the issues the NTSB will be looking at with the Asiana crew, which is what were you doing in the 24 hours before flight? Did you actually sleep?" said Hiatt, president and CEO of the Flight Safety Foundation in Alexandria, Va., which promotes global airline safety.
NTSB has long been concerned about the effects of fatigue not only on airline pilots, but on operators in accidents across all modes of transportation.
Seconds before the Boeing 777 crashed, a member of the flight crew made a call to increase the jet's lagging speed, Hersman said. Then came a warning that the plane was about to stall and cockpit communication that the crew wanted to abort the landing and go back up for another try, she said.
At impact the plane's airspeed was about 106 knots, well below the 137 knots it should have been going as it crossed the runway, Hersman said Monday.
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 that 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.
South Korea's Ministry of Land, Infrastructure and Transport said the 291 passengers included 141 Chinese, 77 South Koreans, 64 Americans, three Canadians, three Indians, one Japanese, one Vietnamese and one person from France.
The two dead passengers have been identified as students from China, 16 and 17 years old, who were scheduled to attend summer camp in California with dozens of classmates. Hospital officials said Sunday that two of the people who remained hospitalized in critical condition were paralyzed with spinal injuries, while another two showed "road rash" injuries consistent with being dragged.
Foucrault, the coroner, said one of the bodies was found on the tarmac near where the plane's tail broke off when it slammed into the runway. The other was found on the left side of the plane about 30 feet (10 meters) away from where the jetliner came to rest after it skidded down the runway.
The flight originated in Shanghai, China, and stopped over in Seoul, South Korea, before making the nearly 11-hour trip to San Francisco.
In the chaotic moments after the landing, when baggage was tumbling from the overhead bins onto passengers and people all around her were screaming, Wen Zhang grabbed her 4-year-old son, who hit the seat in front of him and broke his leg.
Spotting a hole at the back of the jumbo jet where the bathroom had been, she carried her boy to safety.
"I had no time to be scared," she said.
Nearby, people who escaped were dousing themselves with water from the bay, possibly to cool burn injuries, authorities said.
By the time the flames were out, much of the top of the fuselage had burned away. The tail section was gone, with pieces of it scattered across the beginning of the runway. One engine was gone, and the other was no longer on the wing.
Associated Press writers Joan Lowy in Washington D.C., Jason Dearen, Terry Collins, Terry Chea, Lisa Leff and Sudhin Thanawala in San Francisco, David Koenig in Dallas, Louise Watt in Beijing and Hyung-jin Kim in Seoul contributed to this report.
Copyright 2013 The Associated Press.