US officials were combing through the wreckage of an Asiana Airlines Boeing 777 passenger jet in San Francisco Sunday, as they tried to determine why it crashed onto the runway, killing two people and injuring 182 others.

The crash sheared off the plane's landing gear and tore the tail off the fuselage. Large portions of the plane's body were burned out in the fire that then erupted.

But National Transportation Safety Board chief Deborah Hersman said much of the destruction isn't visible in the pictures and footage shown in the news.

"What you can't see is the damage internally. And that is really striking. I think when we look at this accident, we're thankful that we didn't have more fatalities and serious injuries and we have so many survivors," Hersman told CNN's "State of the Union."

Flight 214 originated in Shanghai, and had 307 people on board -- 291 passengers and 16 crew -- after it stopped to pick up passengers in Seoul. The aircraft apparently struck a rocky area at the water's edge short of the airport runway.

Hersman said on ABC her investigation team arrived overnight and "have obtained the cockpit voice recorder and the flight data recorder, and they have been sent back to our labs in Washington. We hope that there is good data, good information on those,"

They also plan to talk to the pilot "in the coming days," to understand what went wrong, she said, on ABC's "This Week."

Part of the landing assistance equipment at the airport was out of service for that runway, but Hersman said that should not have created a dangerous situation.

"There are a number of other tools available to the pilots, some less sophisticated, like the lights, precision approach lights," she told CNN, as well as "some things that are more technologically advanced, (like) things on the airplane that can give you GPS information."

But she said, with or without the instruments, the pilot's expertise is crucial.

"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," she told CNN.

Yoon Young-Doo, the CEO of the Seoul-based Asiana Airlines, had said earlier his company bought the plane in 2006, and that "currently we understand that there are no engine or mechanical problems."

The plane was flown by experienced pilots, and there was no emergency warning ahead of the crash. "Our pilots strictly comply with aviation rules," Yoon said.

He was remorseful as he spoke at a press conference in Seoul. "Please accept my deepest apology," the CEO said, bowing in front of TV cameras.

A four-member South Korean government team was also heading to inspect the site of the accident, officials in Seoul said.

Aboard the flight were 141 Chinese nationals, 77 South Koreans, 61 Americans, one Japanese, three Indians, three Canadians, one French, one Vietnamese, three others of unidentified nationality. There were also 16 crew members, according to Asiana.

Anxious relatives swarmed the airline's headquarters in Seoul, seeking details of the crash and information on the victims.

The two people killed were Chinese passengers sitting in back seats, Asiana's CEO said.

South Korea's transport ministry said separately they were both teenage girls, born in 1996 and 1997.

San Francisco International Airport was closed after the crash but operating normally Sunday.

San Francisco General Hospital said it was treating 34 patients, five of them in critical condition. Other patients were rushed to different area hospitals.

In total, 123 people aboard the flight escaped unharmed, US officials said.

Survivor Elliott Stone told CNN that as the jet came in to land, it appeared to have "sped up, like the pilot knew he was short."

"And then the back end just hit and flies up in the air and everybody's head goes up to the ceiling."

"I saw some passengers bleeding and being loaded onto an ambulance," another passenger, Chun Ki-Wan, told YTN TV in Seoul.

"Everything seemed to be normal before it crash-landed."

The White House said President Barack Obama had been briefed on the incident, noting: "His thoughts and prayers go out to the families who lost a loved one and all those affected by the crash."

South Korean President Park Geun-Hye offered his "deepest condolences to the victims and their relatives," and promised that all government agencies concerned "will join forces to provide all necessary assistance and resources to deal with the disaster."

The twin-engine 777 aircraft is one of the world's most popular long-distance planes, often used for flights of 12 hours or more, from one continent to another.

It was the first fatal crash involving an Asiana passenger plane since June 1993, when a Boeing 737 operated by the carrier crashed into a mountain in South Korea, killing 68.

The US Federal Bureau of Investigation (FBI) said there was no indication that terrorism was to blame for the crash.