A senior pilot in the cockpit of Asiana flight 214 only realised the plane was flying too slowly when it was 200ft (60m) above the ground, officials say.
Investigators are looking at whether automatic equipment in the cockpit could have contributed to the crash.
The Boeing 777 crash-landed at the San Francisco airport on Saturday, killing two passengers and injuring 180.
The pilot at the South Korean plane's controls was about half-way through his training, an official said.
In a press briefing on Tuesday, National Transportation Safety Board Chairman Deborah Hersman cautioned against speculating about the cause of the crash.
Moments from disaster
She also revealed that the airliner's pilots were not tested for drugs or alcohol after the crash, because they do not fall under US regulations.
And she said two flight attendants who had been sitting at the back of the plane were thrown on to the tarmac when its tail broke off as it crashed. They survived but were seriously injured.
Ms Hersman said it was not clear whether the auto-throttle, which keeps the plane's speed, had been fully engaged.
She noted that the pilots were ultimately responsible for controlling air speed.
She said the auto-throttle had been programmed for 157mph (253 km/h).
But an instructor pilot "recognised the auto-throttles were not maintaining speed" seconds before the crash, she added.
About eight seconds before impact, the pilot in control pushed the throttles forward to speed up.
Less than two seconds before the crash, he tried to abort the landing, but it was too late.
Three out of four pilots aboard were in the cockpit as Asiana 214, bound from Incheon in South Korea, approached San Francisco after its 11-hour journey across the Pacific Ocean.
Lee Kang-kuk, who was still completing his training on the Boeing 777 and had never before flown one into San Francisco, was at the controls, Ms Hersman said.
Beside him and in command of the airplane was an instructor pilot, flying in that capacity for the first time.
The plane came in much too shallow. The main landing gear struck a sea wall well short of the end of the runway, and the aircraft's tail was ripped off.
The airplane went into a 360-degree spin before coming to rest to the left of the runway.
The first officer was hospitalised with a cracked rib, and neither of the two pilots were seriously injured.
At least 30 surviving passengers remain in San Francisco hospitals, many with serious spinal injuries.
The two Chinese teenagers who died, Ye Mengyuan and Wang Linjia, had been sitting in the rear of the plane, where many of the most seriously injured passengers were seated, but their bodies were found on the tarmac.
Police are investigating whether one of them survived the crash only to be run over by an emergency vehicle.
Ms Hersman has said airport surveillance video did not conclusively show whether an emergency vehicle had run over one of the students.
A county coroner has said he would need at least two weeks to rule on the death.
Asiana Airlines President Yoon Young-doo arrived in San Francisco on Tuesday to visit victims in hospital and apologise for the crash. He was mobbed by dozens of reporters at the airport.
South Korean President Park Geun-hye has sent a condolence letter to China President Xi Jinping and the families of the two deceased young women, calling the crash "regrettable".
The Boeing 777 has a good safety record, and this is thought to be the first crash involving fatalities.