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WASHINGTON (CNN) -- Neither of the pilots of Comair Flight 5191, which crashed last month in Lexington, Kentucky, had any illegal drugs or alcohol in his blood, the National Transportation Safety Board said Monday.

"An over-the-counter decongestant, pseudoephedrine, was detected at a low level in the first officer's blood," the board said in a written summary of its on-scene investigation into the August 27 crash. Forty-seven passengers and two crew members died in the crash. Only the first officer survived.

The Bombardier CRJ-100 crashed when it attempted to take off from the wrong runway at Blue Grass Airport. The plane, bound for Atlanta, was the third of three airplanes scheduled to take off in the early morning from the airport.

The other two took off without incident from runway 22, which is 7,003 feet long and has lights.

Flight 5191 had also been cleared to taxi and take off from runway 22, but the plane attempted instead to take off from runway 26, which is 3,500 feet long, has no lights and is not approved for nighttime use. It was still dark at 6:07 a.m., when the plane attempted to take off from the shorter runway.

The plane accelerated to about 137 knots, then ran off the end of the runway, drove through a fence and crashed into trees on an adjacent horse farm, the summary said.

"The entire sequence took about 36 seconds. The airplane was destroyed by impact forces and a post-crash fire."

The wreckage was located approximately a third of a mile from the end of the runway. No problems were noted with the engines or flaps, it said.

At the time of the accident, the lone air traffic controller in the tower had overseen two other departures during the prior 20 minutes.

After clearing Flight 5191, "he turned away from the window to perform an administrative task (traffic count); he did not witness the accident, but heard the crash, turned around and saw fire, and immediately activated the emergency response," the report said. "As in all investigations, the group will review the controller's workload and duty schedule and the tower staffing level."

Since the crash, the FAA has staffed the tower with a minimum of two air traffic controllers.

The investigation is not complete. "The group continues to evaluate the pilot actions that led to the attempted takeoff on runway 26," the NTSB said.

In addition, the report said, the board was continuing to study the possible impact of changes to signs and the taxiway that had been instituted during the repaving of the runway.