SOUTH SAN FRANCISCO, Calif. (AP) — The pilots of Asiana Flight 214 relied on automated cockpit equipment to control the jetliner's speed as they landed at San Francisco airport, but realized too late they were flying too low and too slow before the aircraft crashed, investigators said Tuesday.

The new details were not conclusive about the cause of Saturday's crash, but they raised potential areas of focus: Was there a mistake made in setting the automatic speed control, did it malfunction or were the pilots not fully aware of what the plane was doing?

One of the most puzzling aspects of the crash has been why the wide-body Boeing 777 jet came in far too low and slow, clipping its landing gear and then its tail on a rocky seawall just short the runway. The crash killed two of the 307 people and injured scores of others, most not seriously.

Among those injured were two flight attendants in the back of the plane, who survived despite being thrown onto the runway when the plane slammed into the seawall and the tail broke off.

National Transportation Safety Board chairman Deborah Hersman said the autothrottle was set for 157 mph and the pilots assumed it was controlling the plane's airspeed. However, the autothrottle was only "armed" or ready for activation, she said.

Hersman said the pilot at the controls, identified by Korean authorities as Lee Gang-guk, was only about halfway through his training on the Boeing 777 and was landing that type of aircraft at the San Francisco airport for the first time ever. And the co-pilot, identified as Lee Jeong-Min, was on his first trip as a flight instructor.

In the 777, turning the autothrottle on is a two-step process — first it is armed, then it is engaged, Boeing pilots said. Hersman didn't say whether the Asiana's autothrottle was engaged.

Bob Coffman, an American Airlines captain who has flown 777s, said the only way he could think of for Asiana plane to slow as quickly at the NTSB has described would be if somehow the autothrottle has shifted into the idle mode.

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Only moments before the crash did the training captain realize the autothrottle wasn't controlling the plane's speed, Hersman said.

"This is one of the two hallmarks of complexity and challenge in the industry right now," said Doug Moss, an Airbus A320 a pilot for a major U.S. airline and an aviation safety consultant in Torrance, Calif. "It's automation confusion because from what Deborah Hersman said, it appears very likely the pilots were confused as to what autothrottle and pitch mode the airplane was in. It's very likely they believed the autothrottles were on when in fact they were only armed."

Their last second efforts to rev the plane back up and abort the landing failed, although numerous survivors report hearing the engines roar just before impact.

"We just seemed to be flying in way too low. Last couple seconds before it happened the engines really revved into high gear. Just waaah! Like the captain was saying 'oh no, we gotta get out of here.' And then, boom! The back end just lifted up, just really jolted everybody in their seats," said crash survivor Elliot Stone, who owns a martial arts studio in Scotts Valley.

While in the U.S., drug and alcohol tests are standard procedure after air accidents, this is not required for foreign pilots and Hersman said the Asiana pilots had not undergone any testing

A final determination on the cause of the crash is months away, and Hersman cautioned against drawing any conclusions based on the information revealed so far:

Seven seconds before impact, someone in the cockpit asked for more speed after apparently noticing that the jet was flying far slower than its recommended landing speed. A few seconds later, the yoke began to vibrate violently, an automatic warning telling the pilot the plane is losing lift and in imminent danger of an aerodynamic stall. One and a half seconds before impact came a command to abort the landing.

There's been no indication, from verbal calls or mechanical issues, that an emergency was ever declared by pilots. Most airlines would require all four pilots to be present for the landing, the time when something is most likely to go wrong, experienced pilots said. In addition to the two pilots, a third was "monitoring" the landing from a jumpseat, while a fourth was in the rear of the cabin.

"If there are four pilots there, even if you are sitting on a jump seat, that's something you watch — the airspeed and the descent profile," said John Cox, a former US Airways pilot and former Air Line Pilots Association accident investigator.

The Air Line Pilots Association, the world's largest pilots union, criticized Hersman for fueling speculation that the crash is the result of pilot error before all the facts have been determined.

"The NTSB's release of incomplete, out-of-context information has fueled rampant speculation about the cause of the accident," the union said in a statement Tuesday. "The field phase of the investigation is barely three days old, and the pilots on the flight deck, at the controls of the aircraft, had little opportunity to provide vital information as to what exactly happened during the event before disclosing data recorded during the last moments of the flight. "

Hersman said the board was following its usual pattern of trying to be transparent by releasing information as it is known.

By Tuesday afternoon, NTSB interviews with three pilots were complete and the fourth was underway.

In addition, authorities were reviewing the initial rescue efforts after fire officials acknowledged that one of their trucks might have run over one of the two Chinese teenagers killed in the crash. The students, Wang Linjia and Ye Mengyuan, were part of a larger group headed for a Christian summer camp with dozens of classmates.

Asiana President Yoon Young-doo arrived in San Francisco from South Korea on Tuesday morning, fighting his way through a pack of journalists outside customs.

He met with and apologized to injured passengers, family members and survivors. But Yoon said he can't meet with the Asiana pilots because no outside contact with them is allowed until the investigation is completed.

More than 180 people aboard the plane went to hospitals with injuries. But remarkably, more than a third didn't even require hospitalization.

The passengers included 141 Chinese, 77 South Koreans, 64 Americans, three Canadians, three Indians, one Japanese, one Vietnamese and one person from France.

South Korea officials said 39 people remained hospitalized in seven different hospitals in San Francisco.

The flight originated in Shanghai, China, and stopped over in Seoul, South Korea, before making the nearly 11-hour trip to San Francisco.

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Lowy reported from Washington, D.C. Associated Press writers Jason Dearen, Terry Collins, Paul Elias, Lisa Leff and Sudhin Thanawala in San Francisco and Hyung-jin Kim in Seoul also contributed to this report.