A passenger plane carrying 50 people has crashed shortly after taking off from an airport in Kentucky, killing all but one on board.
The Comair CRJ-100 jet, bound for Atlanta, went down in woods about a mile (1.6km) from Lexington's airport at about 0610 (1010 GMT) on Sunday.
Investigators said the plane took off from the wrong runway, a short strip not intended for commercial flights.
The sole survivor, co-pilot James Polehinke, is in critical condition.
Flight 5191 took off from a 3,500ft (1,050-metre) runway instead of the 7,000ft (2,100-metre) runway which it should have used.
A plane of this type needs at least 4,500 feet (1,350 metres) to get fully airborne, experts say.
This is the worst US air accident since November 2001, when an American Airlines plane crashed in Queens, New York, shortly after takeoff from JFK airport, killing 265 people
The plane's "black boxes" - its flight data recorder and cockpit voice recorder - have both been retrieved and sent to Washington DC for analysis.
The "black boxes" have been retrieved and sent for analysis
Peter Goelz, former managing director of the National Transportation Safety Board, said analysts would be trying to determine how the passengers perished: by blunt force trauma, smoke inhalation or burn.
"They'll try and figure out from the position of the victims whether an evacuation was possible," he said.
"From the location of the bodies, they may be able to tell whether an evacuation was attempted."
Mr Goelz added that questions would be asked about the emergency response:
"How quickly did they get there? Did they have the appropriate firefighting materials? Did they put the right stuff on in a timely manner?"
There were marks at the end of the shorter Runway 26 apparently made by the plane, said Debbie Hersman, who is heading the NTSB inquiry into the case.
However, investigators are still trying to ascertain whether the plane had been cleared for takeoff on the longer Runway 22.
Search operations are under way at the site, where a temporary morgue has been set up.
The plane was carrying 47 passengers and three crew members.
Captain Jeffrey Clay and flight attendant Kelly Heyer died in the crash, as did all the passengers.
First officer Mr Polehinke was pulled from the wreckage of the crash by a police officer who was unable to reach any other victims, said police. He is seriously hurt and is being treated at the University of Kentucky Hospital.
The plane exploded into flames on crashing, said Fayette county coroner Gary Ginn, and most of those on board the plane died from burns rather than trauma or smoke inhalation.
"It was a hot fire," he said. "They were taking off, so I'm sure they had a lot of fuel on board."
Family and friends have been gathering at the Atlanta airport.
At a news conference, Comair president Don Bornhorst expressed the "sincere sadness that all at Comair feel".
He said a family care centre had been set up in Lexington to help bereaved relatives.
He said his company had much experience in flying the plane that crashed, and that Comair would co-operate fully with the investigation into the cause.
US President George W Bush was "deeply saddened" by the crash, a White House spokeswoman said.
Comair had bought the aircraft new in 2001 and it had a clean maintenance record, with more than 12,000 cycles of take-offs and landings, Mr Bornhorst told reporters.
Comair is a unit of Atlanta-based Delta Air Lines.
Flights resumed at Lexington's Blue Grass airport about four hours after the crash.