The South Korean jetliner that crash-landed at San Francisco International Airport on Saturday was flying far too slowly to reach the runway and began to stall just before the pilot gunned his engines in a futile effort to abort the landing, the National Transportation Safety Board said Sunday.

The investigation into the crash of the Boeing 777 came to focus more sharply on possible pilot error Sunday as the president of Asiana Airlines ruled out a mechanical failure and federal investigators sought to interview the cockpit crew.

“We’re not talking about a few knots here or there. It was significantly below the 137 knots” required for the approach, NTSB Chairman Deborah A.P. Hersman said in describing data taken from the cockpit and flight data recorders. “We do hope to interview the crew members within the next few days.”

Hersman said the cockpit recorder revealed that seven seconds before impact there was a call to increase the plane’s speed. Three seconds later a “stick shaker” — a violent vibration of the control yoke intended to be a warning to the pilot — indicated the plane was about to stall. Just 11 / 2 seconds before impact, a crew member called out to abort the landing.

Hersman said her agency was a long way — perhaps months — from reaching a conclusion on what caused the crash. But with Asiana insisting there was no mechanical failure, the data from the flight recorders showing the plane far below appropriate speed and the fact that the pilots were controlling the plane in what is called a “visual approach,” the available evidence Sunday suggested the crew was at fault.

On Monday, Asiana spokeswoman Lee Hyomin said that Lee Gang-guk, the pilot in control of the Boeing 777, had little experience flying that kind of plane. She told the Associated Press that it was the pilot’s first time landing in San Francisco and that he had nearly 10,000 hours flying other planes but only 43 hours on the 777.

Two Chinese teenagers were killed and scores of passengers were injured just before noon Saturday when the Boeing 777 airliner struck a sea wall at the end of the runway tail first and skidded about 2,000 feet before catching fire.

Authorities said the two girls were thrown from the plane onto the runway, and the Associated Press reported Sunday night that the San Francisco coroner was investigating whether one died after being run over by an emergency vehicle rushing to the plane.

At least eight passengers remained in critical condition at two hospitals Sunday, officials said. Six of them were at San Francisco General Hospital, where the chief of trauma surgery, Margaret Knudson, said that some of the 53 patients taken to the emergency room suffered minor burns or injuries caused by seat belts or from slamming into other seats. Those still in critical condition had head injuries, internal bleeding or fractured spines.

“We are used to these types of injuries, just not used to seeing them all at once,” Knudson said.

Hersman said the seven-year-old plane was equipped with current navigation tools to assist landings, including recent advances in GPS technology.

“A lot of this is not about the plane telling him, it’s about the pilot’s recognition of what’s going on . . . to be able to assess what’s happening and make the right inputs to make sure they’re in a safe situation,” Hersman said. “That’s what we expect from pilots. We want to understand what happened in this situation.”

Pilot error investigated in Flight 214 crash;

In Seoul, from where Flight 214 departed for the transpacific flight with 307 people on board, the president of Asiana Airlines apologized Sunday for the crash.

“For now, we acknowledge that there were no problems caused by the 777-200 plane or engines,” Yoon Young-doo said at the company’s headquarters.

Chinese state media identified the two 16-year-olds who died in the crash as Ye Mengyuan and Wang Linjia, high school students from China’s eastern Zhejiang province. They were among a group of 70 students and teachers from three Chinese schools.

The jet was the first large plane to go down in U.S. airspace since November 2001, when an American Airlines Airbus A300 crashed on takeoff from New York’s John F. Kennedy International Airport, killing all 260 people aboard as well as five people on the ground.

With dozens of witnesses, including several other airline pilots awaiting takeoff, the cause of the Flight 214 crash should be easier to resolve than the disasters whose flight numbers became iconic in aviation history: Air Florida Flight 90 in 1982, Pan American Flight 103 in 1988 and TWA Flight 800 in 1996. The Pan Am and TWA flights exploded in midair, and Air Florida crashed into Washington’s 14th Street bridge on takeoff. Their pilots did not survive.

Hersman, in conversations with reporters Sunday, underscored that the NTSB would conduct its work in a methodical, cautious manner.

“It’s a little bit early to be drawing conclusions. We really prefer to base statements on fact, want to establish the facts and let the facts guide us in our work,” she said. NTSB teams “will be looking at aircraft operations, at human performance, survival factors, and we’ll be looking at the aircraft,” she added. “We’ll be looking at power plants, systems and structures.”

Passengers described a normal approach that was punctuated by a sudden acceleration of the engines just as they expected the wheels to touch down. That conformed to the observations of witnesses who said the plane struggled to reach the beginning of the runway.

Witnesses said that the plane’s tail struck the ground first and that the aircraft braked suddenly and spun around. They said the plane did not appear to catch fire until it came to a halt.

The plane’s tail snapped off on contact with the ground, suggesting that the pilot may have approached the runway with the plane’s nose higher than normal.

The runway begins at the edge of San Francisco Bay, separated from the water by a stone sea wall. Debris from the plane was spread from the sea wall along the runway to the spot where the aircraft came to a stop.

The tail fin, the two small tail wings that had been joined to it and landing gear were strewn on the runway, closer to the sea wall than to the plane’s final location.

The Boeing 777 is considered a reliable aircraft with a good flight history. It was designed for long-range trips, including many transatlantic and transpacific flights.

Asiana configured the Boeing 777 used in Flight 214 to carry 303 people. With crew included, 307 were on board when the flight took off from Seoul just before 5 p.m. local time Saturday (4 a.m. Eastern time) for the trip across the Pacific.

Asiana is South Korea’s second-largest carrier, after Korean Air. It has recently tried to expand its presence in the United States and joined the Star Alliance, which is anchored in the United States by United Airlines. Asiana has a fleet of 79 aircraft, including a dozen Boeing 777s. It flies to 23 countries and 71 cities.

Lori Aratani in Washington; Del Quentin Wilber, Janine Zacharia and Gregory Thomas in San Francisco; and Yoonjung Seo in Seoul contributed to this report.