A passenger plane which crash landed at San Francisco International Airport, killing at least two people, had no engine or mechanical problems, according to the airline's boss.
Around 180 people were injured after the South Korean Asiana Airlines jet slammed into the runway, broke off its tail and then the aircraft caught fire.
The Boeing 777, which flew from Seoul to the US west coast city, was carrying 291 passengers and 16 crew members.
Some 181 people were taken to nine hospitals - 49 of whom are in a serious condition.
The dead were female Chinese teenagers, aged 16, from the same school who were seated at the back of the plane. They were found outside the aircraft.
The victims from Jiangshan city, in Zhejiang Province, south of Shanghai, were on a school trip.
Asiana Airlines chief executive Yoon Young-Doo said: "We purchased this airplane in March 2006... currently we understand that there are no engine or mechanical problems."
Mr Yoon said the crew had made an in-flight broadcast as usual, "asking passengers to buckle up for landing. There was no emergency alarm".
He declined to comment directly on whether the crash on Flight 214 was due to pilot error.
But he said the two pilots had been fully trained in compliance with Korean regulations and had more than 10,000 flying hours of experience between them.
Mr Yoon also declined to say whether he believed the crash could have been caused by an error by air traffic controllers at San Francisco.
Terrorism had been ruled out as a cause of the accident, the FBI confirmed.
Eyewitnesses reported seeing the plane's tail section break off during the crash. South Korea's transportation ministry said the tail hit the runway before it crashed.
A fire started when the plane landed and passengers were forced to use emergency inflatable slides to escape the aircraft, witnesses added.
Parts of the tail and the landing gear were strewn across the runway. TV footage showed the top of the fuselage had been burned away and one engine appeared to have broken off.
David Eun, believed to be a passenger on the aircraft, posted on Twitter: "I just crash landed at SFO. Tail ripped off. Most everyone seems fine. I'm ok. Surreal."
He added: "Fire and rescue people all over the place. They're evacuating the injured."
Federal Aviation Administration spokeswoman Laura Brown said the Boeing 777 was supposed to land on runway 28 left at San Francisco International Airport.
She said the sequence of events was still unclear, but it appeared the plane landed and then crashed.
Debbie Hersman, chairman of the Transport Safety Board, said: "Teams are going to be focused on operations, human performance, survival factors, the airport, airport operations and they are going to be focussing on the aircraft - the systems, the structures and the power plants.
"It's still too early for us to tell (what happened).
A statement from San Francisco General Hospital said 10 patients from the crash - eight adults and two children - were in a critical condition.
Asiana is second in size to the South Korean national carrier Korean Air.
It has recently tried to expand its presence in the United States, and joined the Star Alliance, which is anchored in America by United Airlines.