A burned Asiana Airlines Inc. plane sits on the runway after it crashed landed at San Francisco International Airport (SFO) in San Francisco, California, U.S., on Saturday, July 6, 2013. A Boeing Co. 777 operated by Asiana Airlines Inc. on a flight from Seoul crashed while landing at San Francisco International Airport, killing two and burning as passengers plummeted down emergency slides. Photographer: David Paul Morris/Bloomberg via Getty Images
(CNN) -- Investigators gathered critical clues in San Francisco on Sunday in hopes of solving the mystery surrounding the deadly crash landing of Asiana Airlines Flight 214.
Both flight data recorders have been recovered, the National Transportation Safety Board said, from wreckage left by Saturday's tragedy that left two 16-year-old passengers dead.
Survivors and witnesses reported the 7-year-old Boeing 777 appeared to be flying too low as it approached the end of a runway near the bay.
"Stabilized approaches have long been a safety concern for the aviation community," NTSB Chairman Deborah Hersman told CNN on Sunday, saying they represent a significant threat. "We see a lot of runway crashes."
"We want to understand what was going on with this crew so we can learn from it," Hersman said.
Hersman said her team hopes to interview the pilots in the coming days.
Internal damage to the plane is "really striking," she said, and officials are thankful there weren't more deaths.
Nothing, including pilot error, has been ruled out as a possible cause of the crash, investigators said. The recorders have already arrived at an NTSB lab in Washington for analysis.
Teen girls Ye Mengyuan and Wang Linjia, both Chinese nationals, were killed in the crash, Asiana Airlines said Sunday. There were 291 passengers and 16 crew members aboard the two-engine jet, which had flown a 10-hour direct flight from Seoul, South Korea.
"The tail of the Asiana flight hit the runway and the aircraft veered to the left out of the runway," said Choi Jeong-ho, head of South Korea's Aviation Policy Bureau.
Expert: The plane 'should never have been close to the seawall'
Airport technology called the Instrument Landing System, or ILS -- which normally would help pilots correctly approach the runway -- was not operating at the time, according to a Federal Aviation Administration bulletin.
"There are a lot of systems that help support pilots" as they fly into busy airports, Hersman said. Some of these systems alert the pilots. "A lot of this is not necessarily about the plane telling them" that something may be wrong, she said. "It's also about the pilot's recognition of the circumstances and what's going on. So for them to be able to assess what's happening and make the right inputs to make sure they're in a safe situation -- that's what we expect from pilots."
The ILS integrates with the aircraft's cockpit to trigger a audible warning, retired 777 pilot Mark Weiss told CNN. "You hear a mechanical voice that says, 'too low, too low, too low.'" The ILS is "nice to have," Weiss said, "but it's not critical on the 777." There are redundant systems aboard the aircraft that would provide similar warnings if the plane was coming in too low, said Weiss, who has landed 777s hundreds of times.
Weiss said he's perplexed by the details surrounding the crash landing. If the pilot was somehow unaware the plane was coming in too low, Weiss wonders why another member of the flight crew didn't speak up and warn him.
The pilot operating the aircraft was a veteran who had been flying for Asiana since 1996, the airline said. Evidence in the investigation will include data that show what action the pilots took during the approach to the airport.
Although temperatures were mild on the runway Saturday, a London crash landing of a British Airways 777 in 2008 raised suspicions about ice contributing to the San Francisco tragedy.
Investigators believe the UK incident was caused by ice forming in the fuel system as the plane flew through cold air over Siberia en route to the UK. Seventeen people were hurt in that crash.
"Even if the landing temperature was 65 degrees (Saturday), if they were at 10,000 feet shortly before, it potentially could have been a problem," said Weiss. "But not necessarily. It's something they want to take a look at."
San Francisco Fire Chief Joanne Hayes-White said that when crews arrived, "some of the passengers (were) coming out of the water. But the plane was certainly not in the water."
Survivors reported hearing no warning from the cockpit before the plane slammed onto the edge of the runway, severing the plane's tail and sending the fuselage spinning on its belly. The crash landing blew a fireball and clouds of smoke into the sky.
Passengers scrambled to exit a crash scene that one survivor described as "surreal."
On the runway, medics found the bodies of the two Chinese teens lying next to burning wreckage. Remarkably, the other 305 people on the plane survived. Passengers included 70 Chinese students and teachers who were headed to summer camp in the United States, China's state-run Xinhua news agency reported.
"We're lucky there hasn't been a greater loss of life," Hayes-White said. When rescuers arrived, they found some passengers coming out of the water, she said.
"There was a fire on the plane, so the assumption might be that they went near the water's edge, which is very shallow, to maybe douse themselves with water," Hayes-White said.
While 182 of them were taken to hospitals with injuries ranging from spinal fractures to bruises, another 123 managed to escape unharmed.
Some jumped out or slid down emergency chutes with luggage in hand.
South Korean investigators will work alongside U.S. investigators. Choi said it could take up to two years to learn exactly what caused the crash. Asiana CEO and President Yoon Young-doo said there was no engine failure, to his knowledge. "The company will conduct an accurate analysis on the cause of this accident and take strong countermeasures for safe operation in the future with the lesson learned from this accident," Yoon said.
Statistically, 2012 was the safest year in terms of aviation accidents worldwide since 1945, according to the Aviation Safety Network.
Data show that there were 23 fatal airliner accidents, which caused 511 deaths, according to ASN stats. That's well below the 10-year annual average of 34 accidents and 773 fatalities.
Survival rates have improved due to better staff training and safety advances during the 1980s and 1990s, according to the group.