Asiana CEO and President Yoon Young-doo has said there was no engine failure, to his knowledge. South Korean investigators will work alongside U.S. investigators.
The NTSB has ruled out weather as a problem and said that conditions were right for a "visual landing."
But investigators are looking into whether construction at the airport may have played a role.
Construction to extend a runway safety area temporarily shut off the so-called glide slope system, which is one of several options pilots have to help them land planes safely, Hersman said.
Internal damage to the plane is "really striking," she said, and officials are thankful there weren't more deaths.
Survivors of the crash were being treated Sunday for injuries ranging from paralysis to "severe road rash."
In all, 182 people were hospitalized and 123 others walked away from the crash landing. The number who emerged unscathed prompted the city's fire chief to describe it as "nothing short of a miracle."
On the runway, medics found the bodies of the two teen girls lying next to burning wreckage. They were identified by Asiana Airlines as Ye Mengyuan and Wang Linjia, both Chinese nationals.
"We were expecting a lot of burns," said Dr. Margaret Knudson, San Francisco General Hospital's chief of surgery. "But we didn't see them."
At San Francisco General, 17 survivors remained hospitalized, six of them in critical condition.
Many of the injured said they were sitting toward the rear of the aircraft, said Knudson. Several suffered abdominal injuries and spine fractures, some of which include paralysis and head trauma, she said.
Many patients also were treated for "severe road rash," which suggests "that they were dragged," Knudson said.
The conditions of victims at other hospitals were not made public Sunday.
Survivors and witnesses reported the 7-year-old airliner appeared to be flying too low as it approached the end of a runway near the bay.
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.
The ILS integrates with the aircraft's cockpit to trigger an audible warning, consultant and 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.
The pilot operating the aircraft was a veteran who had been flying for Asiana since 1996, the airline said.
Survivors reported hearing no warning from the cockpit before the landing.
Passengers scrambled to exit a crash scene that one survivor described as "surreal."
When rescuers arrived, they found some passengers coming out of the water, said city fire chief Joanne Hayes-White.
"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," she said.
Some jumped out or slid down emergency chutes with luggage in hand.
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.