The fatalities, teenagers Ye Mengyuan and Wang Linjia, were among 35 Chinese students headed to California to attend West Valley Christian School's summer church camp, the school said on its website.
Both of the girls had been seated in the rear of the plane, which suffered significant structural damage, Hersman said.
Investigators said they were looking into reports that one of the girls may have been run over by an emergency vehicle. "We are still looking at this issue," she said. "The coroner has not yet determined the cause of death and so we want to make sure we have all the facts before we reach any conclusions."
Pilot's flight record
Lee Kang-kuk, the pilot who was in the captain's seat of Flight 214, had flown from Seoul to San Francisco several times between 1999 and 2004, the airline said.
But Saturday marked his first time landing a Boeing 777 at San Francisco International Airport and was the ninth time he had flown the model, with 43 hours at the controls, the airline said. He has about 10,000 hours as a pilot, Asiana said.
Hersman, who has discouraged speculation about whether the crew bore responsibility for the crash, downplayed the significance of the pilot's experience in her New Day interview.
"It's not unusual for crew to change aircraft types," she said. And with air crews flying all around the world, it's not unusual for pilots to fly into unfamiliar airports for the first time either.
She said it's important for the two pilots in charge of the aircraft during the "very risky" landing phase to work closely together, and while she said investigators have no evidence of cockpit communications problems, it's something investigators will be looking at.
Mary Schiavo, a former inspector general for the U.S. Department of Transportation, said video and other data related to the crash suggest the crew "lost situational awareness" while approaching the airport.
"They're low and slow, and that's a problem," Schiavo said.
All four pilots have been interviewed by NTSB and South Korean investigators, said Choi Jeong-ho, the head of South Korea's Aviation Policy Bureau.
"We cannot reveal what's been said as it is an ongoing investigation," Choi said.
The pilots represented two teams -- a crew and a relief crew, said Hersman.
"I think it really is too early to conclude pilot error, because there is so much that we don't know," she told CNN's Wolf Blitzer.
The cockpit voice recorder -- which has been flown to Washington for analysis -- contains conversations between the pilots that were carried out in a combination of English and Korean, she said.
Hersman said that in most airplane crashes, investigators rarely find a single explanation for what went wrong.
"In most of our investigations, we find that it's not just one thing, it really is a combination of factors that lead to an accident," she said.
While weather has been ruled out as a factor, other factors officials are investigating include whether construction at the airport may have played a role, Hersman said Sunday.
Work to extend a runway safety area required the temporary shutdown of a system designed to help pilots land planes safely, she said.