Investigators probing the Asiana Airlines plane crash in San Francisco began interviewing cockpit crew of the Boeing 777 on Monday amid mounting indications that pilot error may have caused the fatal accident.

Two teenage Chinese girls were killed and more than 180 people were injured when the Asiana flight from Seoul clipped a seawall short of the runway and went skidding out of control on its belly, shredding the tail end of the plane and starting a fire.

Deborah Hersman, head of the National Transportation Safety Board (NTSB), said the aircraft's four-man flying crew were being quizzed as it emerged the plane had been flying well below the recommended speed for landing when it crashed at San Francisco International Airport on Saturday.

"We will determine exactly what happened, when it happened and how it happened, if it was consistent with their process or procedures or if there is any deviation," she said.

Flight data showed the plane had been traveling at approximately 106 knots at impact -- sharply lower than the target speed necessary for landing.

"137 knots is the speed that they want to have when they cross the threshold of the runway," Hersman said, noting that the aircraft crew were tasked with ensuring the correct speed of the plane.

"The crew is responsible to make a safe approach to the airport. (Air traffic controllers) are not responsible for speed management on the aircraft."

Asiana confirmed that the pilot of the aircraft, 46-year-old Lee Kang-Kuk, was being trained on operating the Boeing 777.

The airline said Lee had just 43 hours of experience in piloting the popular passenger aircraft, although he had accumulated more than 9,000 hours of flight time experience on other planes.

Asiana CEO Yoon Young-Doo described media reports that pilot error may have caused the tragedy as "intolerable," calling it a "matter of speculation."

Hersman later told CNN it was too soon to blame the accident on human error.

"It really is too early to conclude a pilot error because there is so much that we don't know," she said.

"We have to understand what these pilots knew, we also need to look at how they were flying the aircraft -- were they handflying the airplane? Were they relying on auto-pilot or some combination of the two? "We've seen some of these challenges in previous investigations with respect to automation and so we want to make sure that we fully understand it before we reach any conclusions."

Chinese state media meanwhile identified the two dead passengers as Ye Mengyuan, 16, and Wang Linjia, 17, high school classmates from eastern China's Zhejiang province.

One of the girls may have been run over by an airport fire engine rushing to the scene, San Francisco Fire Chief Joanne Hayes-White told reporters on Monday.

"That is a possibility," Hayes-White said.

San Mateo County Coroner Robert Foucrault earlier told the San Francisco Chronicle that the other girl appeared to have died from injuries suffered as she was hurled out of the plane.

The two friends were coming to visit Stanford University, just south of San Francisco, and to attend a summer camp at a local Christian school, the Chronicle reported.

Asiana Flight 214, which originated in Shanghai and stopped in Seoul, had 291 passengers and 16 crew members aboard.

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

On Monday, a group of 11 South Korean passengers who were aboard Flight 214 returned home, including a 28-year-old woman who was traveling with her husband for their first wedding anniversary.

"I feel so much pain both physically and mentally," the woman told Yonhap news agency.

"My whole body aches."

Another passenger who arrived at Seoul's Incheon airport was carried on a stretcher to a waiting ambulance, Yonhap reported.

The twin-engine Boeing 777 is used for long distance flights.

According to aviation safety databases, the two dead teens are the 777's first fatalities in 18 years of service.

It was the first deadly Asiana passenger plane crash since June 1993, when one of its Boeing 737s slammed into a mountain in South Korea, killing 68.

Asiana's share price ended down nearly six percent on Monday as investors digested the impact of the accident.

Huge insurance payouts to victims and for the aircraft could raise future premiums and increase the company's financial burdens.

Aviation experts, however, said the damage to its business could be limited, as South Korea's number two airline has spent years building a reputation for safety and quality.