The lone air traffic controller on duty on the morning of the Kentucky air crash had cleared the jet for takeoff, but turned away as the plane veered down the wrong runway.

Comair Flight 5191 crashed killing 49 passengers after taking off from the wrong runway while the controller was distracted by "administrative duties," investigators said.

The plane struggled to get airborne on the shorter runway and crashed in a nearby field, bursting into flames.

The only survivor was first officer James Polehinke, who is still in a critical condition.

As investigators revealed the fatal oversight, the Federal Aviation Administration separately acknowledged violating its own policies when it assigned only one controller to the Lexington airport's tower.

The two developments last night began to explain factors that may have contributed to last Sunday's crash, the US's worst airline disaster since 2001.

Yesterday families of the victims toured the crash site before a memorial service. The field where Flight 5191 crashed is still scattered with wreckage and the grass is stained with engine fuel. Trees by the runway have both bright green leaves and charred black ones.

The few personal items gathered at the site have been covered with transluscent plastic.

Authorities said the pilot tried to take off from a small 1,050-meter runway instead of an adjoining full sized one.

The air traffic controller, a 17-year veteran at the airport who has not been publicly identified, had an unobstructed view of the runways and had cleared the aircraft for takeoff from the longer runway, said National Transportation Safety Board member Debbie Hersman.

Then, "he turned his back to perform administrative duties," Hersman said. "At that point, he was doing a traffic count."

The FAA admitted it had violated a policy, outlined in a November 2005 directive, requiring control tower observations and radar approach operations be handled by separate controllers.

FAA spokeswoman Laura Brown said the controller at the Lexington airport had to do his own job keeping track of airplanes on the ground and in the air up to a few miles away as well as radar duties.

Polehinke was flying the plane when it crashed, but it was the flight's captain, Jeffrey Clay, who taxied the aircraft onto the wrong runway, Hersman said. Clay then turned over the controls to Polehinke for takeoff, the investigator said.