(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.
Airport technology called the Instrument Landing System, or ILS -- which normally would have helped 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."
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."
Evidence in the investigation will include data that show what action the pilots took during the approach to the airport. The pilot operating the aircraft was a veteran who had been flying for Asiana since 1996, the airline said.
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.
CNN's Faith Karimi and Aaron Cooper contributed to this report.
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