Authorities also reviewed the initial rescue efforts after fire officials acknowledged that one of their trucks may have run over one of the two Chinese teenagers killed in the crash at San Francisco International Airport. The students were the accident's only fatalities.
National Transportation Safety Board Chairman Deborah Hersman said investigators watched airport surveillance video to determine whether an emergency vehicle hit one of the students. But they have not reached any firm conclusions. A coroner said he would need at least two weeks to rule in the matter.
The students had been in the rear of the aircraft, where many of the most seriously injured passengers were seated, Hersman said.
The NTSB also said part of the jet's tail section was found in San Francisco Bay, and debris from the seawall was carried several hundred feet down the runway, indicating the plane hit the seawall on its approach.
Investigators have said Flight 214 was flying "significantly below" its target speed during approach when the crew tried to abort the landing just before the plane smashed onto the runway. Authorities do not know yet whether the pilot's inexperience with the Boeing 777 and landing it at San Francisco's airport played a role.
The airline acknowledged Monday in Seoul that the pilot at the controls had flown that type of plane for only a short time and had never before landed one at that airport.
Asiana spokeswoman Lee Hyomin said pilot Lee Gang-guk had logged nearly 10,000 hours operating other planes but had only 43 hours in the 777, a plane she said he was still getting used to.
It's not unusual for veteran pilots to learn about new aircraft by flying with more experienced colleagues. Another pilot on the flight, Lee Jeong-min, had 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 Jeong-min was the deputy pilot helping Lee Gang-guk get accustomed to the 777, according to Asiana Airlines.
It was unclear whether the other two pilots were in the cockpit, which in the Boeing 777 typically seats four. But that would be standard procedure at most airlines at the end of a long international flight.
NTSB lead investigator Bill English said pilot interviews were going slowly because of the need for translation. The interviews began only after agents from the Korean Aviation and Rail Accident Investigation Board arrived from South Korea.
New details of the investigation have also raised questions about whether the pilots may have been so reliant on automated cockpit systems that they failed to notice the plane's airspeed had dropped dangerously low, aviation safety experts and other airline pilots said.
Information gleaned from the Boeing 777's flight-data recorders revealed a jet that appeared to be descending normally until the last half-minute before impact.
The autopilot was switched off at about 1,600 feet as the plane began its final descent, according to an account of the last 82 seconds of flight provided by Hersman.
Over the next 42 seconds, the plane appeared to descend normally, reaching about 500 feet and slowing to 134 knots (154 mph), a 777 pilot for a major airline familiar with Hersman's description told The Associated Press. The pilot spoke on the condition of anonymity because his company had not authorized him to speak publicly.
But something went wrong during the following 18 seconds. The plane continued slowing to 118 knots (136 mph), well below its target speed of 137 knots (158 mph) that is typical for crossing the runway threshold. By that time, it had descended to just 200 feet.
Eight seconds later, with the speed still falling, Hersman said, the throttles were moved forward, an apparent attempt by the pilot to increase speed. But it was too little, too late.
Five seconds later, at 50 per cent power, speed began to increase.
A key question raised by the NTSB's account is why two experienced pilots — the pilot flying the plane and another supervising pilot in the other seat — apparently didn't notice the plane's airspeed problem.
Part of the answer to that question may lie in whether the pilot flying, after switching off the autopilot, still had the plane's autothrottle engaged during the descent.
Aviation safety experts have long warned that an overreliance on automation is contributing to an erosion of pilots' stick-and-rudder flying skills. It's too soon to say if that was the case in the Asiana crash, but it's something NTSB investigators will be exploring, they said.
"It sounds like they let the airplane get slow and it came out from under them," said John Cox, a former US Airways pilot and former Air Line Pilots Association air crash investigator.
"There are two real big questions here: Why did they let the airplane get that slow, and where was the non-flying pilot, the monitoring pilot, who should have been calling out 'airspeed, airspeed, airspeed,' " Cox said.
More than 180 people aboard the plane went to hospitals with injuries. But remarkably, 305 of 307 passengers and crew survived, and more than a third didn't even require hospitalization. Only a small number were badly hurt.
The passengers included 141 Chinese, 77 South Koreans, 64 Americans, three Canadians, three Indians, one Japanese, one Vietnamese and one person from France. Asiana President Yoon Young-doo planned to leave for San Francisco later Tuesday to visit hospitalized passengers, according to Asiana spokeswoman Lee Hyomin.
Twenty-three South Koreans have so far left for San Francisco to visit their injured family members and relatives since the crash, according to South Korea's Ministry of Land, Infrastructure and Transport.
After the crash, three firefighters — and two police officers without safety gear — rushed onto the plane to help evacuate trapped passengers, including one who was trapped under a collapsed bulkhead.
They had gotten everyone off the craft except one elderly man, who was in his seat, moaning and unable to move.
"We were running out of time," San Francisco Fire Department Lt. Dave Monteverdi recalled Monday at a news conference. "The smoke was starting to get thicker and thicker. So we had no choice. We stood him up and amazingly, he started shuffling his feet. That was a good sign...we were able to get him out and he was pretty much the last person off the plane."
The two dead passengers were identified as 16-year-old students from China who were scheduled to attend summer camp in California with dozens of classmates.
One of their 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 metres) 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.
NTSB investigators are also sure to examine whether pilot fatigue played a role in the accident, which occurred after a 10-hour nighttime flight. As is typical for long flights, four pilots were aboard, allowing the crew to take turns flying and resting. But pilots who regularly fly long routes say it's difficult to get restful sleep on planes.
The accident occurred in the late morning in San Francisco, but in Seoul it was 3:37 a.m.
"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."
The two teenagers killed in the crash were close friends and top students.
Wang Linjia showed talent in physics and calligraphy; Ye Mengyuan was a champion gymnast who excelled in literature. Both were part of a trend among affluent Chinese families willing to spend thousands of dollars to send their children to the U.S. for a few weeks in the summer to practice English and hopefully boost their chances of attending a U.S. college — considered better than China's alternatives by many Chinese families.
The girls posted their last messages on their microblog accounts Thursday and Friday. The last posting from Wang said simply, "Go."
Lowy reported from Washington.
Associated Press writers Jason Dearen, Terry Collins, Paul Elias, Lisa Leff and Sudhin Thanawala in San Francisco, Gillian Wong and Didi Tang in Beijing, and Hyung-jin Kim in Seoul also contributed to this report.