The only air traffic controller on duty when a plane crashed in Kentucky on Sunday, killing 49 people, had his back turned at the time, investigators said.
The crash was the worst in the US for five years
He failed to notice the plane had taken the wrong runway as it tried to take off, as he had turned away "to perform administrative duties", officials said.
The runway was too short and the Comair CRJ-100 crashed shortly after take-off.
The Federal Aviation Administration admitted it broke its own rules in putting only one controller on duty.
"This controller did not have visual contact with the aircraft," said Debbie Hersman, a member of the National Transportation Safety Board.
The man was not named but had been working at the Lexington airport for 17 years and was fully qualified, Ms Hersman said.
At the crucial moment "he was doing a traffic count", she said.
The plane's pilots were apparently unaware that take-off procedures at the airport had changed only a week earlier.
They used a 3,500ft (1,050m) runway, meant for small aircraft, instead of the one next to it, which was twice as long.
Experts said the plane needed at least 5,000 ft (1,524m) of runway.
The air traffic controller had cleared the aircraft for take-off from the longer runway, but turned away and therefore failed to see it advance up the wrong strip, Ms Hersman 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.
Runway 26 was not designed for use by commercial jets
The only survivor of the crash was the co-pilot, first officer James Polehinke, who was at the controls at the time of the crash.
He remains in a critical condition in hospital.
Doctors said it was miraculous that he was alive, as he had broken bones throughout his body, including in his face and spine.
His mother, Honey Jackson, urged people not to blame him for the crash.
"He could die at any moment," she said.
The crash is the worst US air accident since November 2001, when an American Airlines plane came down in Queens, New York, shortly after take-off from JFK airport, killing 265 people.