Data showed the Comair CRJ-100 jet took off from a short strip not intended for commercial flights, the officials said.

The aircraft, bound for Atlanta, went down in woods about a mile (1.6km) from Lexington's Blue Grass Airport at about 0610 (1010 GMT) on Sunday.

The plane was carrying 50 people and the sole survivor, co-pilot James Polehinke, is in critical condition.

The plane's "black boxes" - its flight data recorder and cockpit voice recorder - have both been retrieved and sent to Washington DC for analysis.

Clearance question

Investigators will try to determine why flight 5191 took off from a 3,500ft (1,050-metre) runway instead of the 7,000ft (2,100-metre) runway it should have used.

A plane of this type needs at least 4,500 feet (1,350 metres) to get fully airborne, experts say.

The "black boxes" have been retrieved and sent for analysis

Officials said it was too early to tell whether the flight had been cleared for take-off from the shorter Runway 26.

This is the worst US air accident since November 2001, when an American Airlines plane crashed in Queens, New York, shortly after take-off from JFK airport, killing 265 people

Debris search

There were marks at the end of Runway 26 apparently made by the plane, said Debbie Hersman, who is heading the National Transportation Safety Board inquiry into the case.

Search operations were under way at the site, where a temporary morgue was 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 James 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.

President 'saddened'

The plane exploded on impact, said Fayette county coroner Gary Ginn, and most of those on board the plane died from burns.

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.

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 airline.

Source:BBC