LEXINGTON, Ky. (AP) _ What is known is that a string of mistakes preceded the deadly crash of Comair Flight 5191, but what is less clear is which one was the crucial turning point.
Did the fatal error occur when the first officer started the takeoff on the wrong runway, too short for the twin-engine jet, even though the flight crew noticed it had no lights?
Was the problem the airport itself? The captain had to follow an unfamiliar taxi route that had been changed by a repaving project just a week earlier.
Could the air traffic controller have stopped the crash? He had only two hours of sleep before working the overnight shift, and he had turned his back to do administrative work just before takeoff.
Or, was it a decision by the tower manager to break the federal rule that two controllers should be working there at all times? Or even earlier, when the airport built intersecting runways rather than parallel ones?
``It just breaks your heart if you're an investigator because you know if the chain is broken at any step of the way, lives are not going to be lost,'' said Peter Goelz, a former managing director at the National Transportation Safety Board.
If past crashes are any indication, there will be no simple explanation for why Flight 5191 crashed while trying to take off from Blue Grass Airport, killing 49 people. Only the first officer survived, but he was badly injured and remains in serious condition.
Instead, the answer may lie in a web of small mistakes, nearly all of them preventable.
NTSB officials largely wrapped up their site work by Thursday, but it could be a year before the agency issues its final opinion about the cause of the crash.
During briefings, there was little mention of technical problems with the aircraft or poor weather, but potential human errors were identified daily.
It's the ``Swiss cheese'' theory of human error, credited to psychology professor James Reason. There may be numerous safeguards to prevent a crash, but like Swiss cheese, they each have a few holes. When all the holes line up, disaster can strike.
Technological advancements keep making airports and airplanes safer, but technology can't eliminate all human error, said Bill Waldock, aviation safety professor at Embry-Riddle Aeronautical University at Prescott, Ariz.
``It's the very nature of the human being that you're going to have mistakes,'' Waldock said. ``While we have a fairly good ability to catch those mistakes before they become catastrophic, every now and then one of them slips through.''
Even before the NTSB's final report, the crash could have an effect on the aviation industry.
Changes in control tower scheduling, extra preflight checkoffs for pilots and runway alert systems have all been advocated.
Some control tower operators have complained about declining staffing levels at airports. Federal rules require eight hours off between shifts, but they say that's not enough for controllers to be well rested.
``It's a 300-mile-an-hour chess game, and they don't want to spend the money,'' said Terry Housh, a former air traffic controller at several airports.
Federal Aviation Administrator Marion Blakey said that the Blue Grass controller had the required time off between shifts, and that it was his responsibility to know whether he'd had enough sleep.
``We expect that when our staff report for work that they determine that they are fit for duty,'' Blakey said.
As for the airplane crew, John Nance, a pilot and aviation safety consultant, noted that it's standard for pilots to cross-check runway headings on their compass before taking off. Nance said he can't imagine how the crew members turned onto the wrong runway, headed in the wrong direction, if they had done that.
``Probably the words 'wrong runway' never popped into their heads,'' Nance said.
``I just can't imagine as a pilot how you could blast off in the darkness like that,'' Waldock said. ``Even if you've been up for three days with no sleep, I can't imagine doing something like that.''