The crash killed 49 people - everyone on board except first officer James Polehinke, who was in critical condition today.
The jet struggled to get airborne and crashed in a field after taking off on Sunday from a 1,050-metre long runway instead of an adjoining one that was twice as long.
Experts said the plane needed at least 1,500 metres for takeoff.
The air traffic controller had an unobstructed view of the runways and had cleared the aircraft for take-off from the longer runway, said National Transportation Safety Board member Debbie Hersman.
Then, "he turned his back to perform administrative duties," Ms Hersman said. "At that point, he was doing a traffic count."
Earlier the Federal Aviation Administration acknowledged violating its own policies when it assigned only one controller to the airport tower that morning.
The policy is outlined in a 2005 directive requiring that control tower observations and radar approach operations be handled separately.
FAA spokeswoman Laura Brown said the controller had to do his own job - keeping track of airplanes on the ground and in the air up to a few kilometres away - as well as radar duties.