The pilots of a Comair commuter jet who turned onto the wrong runway may have been confused by an advisory issued earlier that the lights on the proper runway were out, aviation sources said on Monday.

National Transportation Safety Board investigators said the plane was cleared by controllers to take off from the longer of two runways but turned onto the wrong one and crashed, killing 49 of the 50 people aboard.

"The planning discussions with air traffic controllers and the flight crew were about a takeoff from runway 22," a 7,000-foot (2.1 km) runway suited for jets at Lexington's Blue Grass Airport, Safety Board member Debbie Hersman said.

Instead, the Comair jet, bound for Atlanta before dawn on Sunday in a light rain, took runway 26. That runway is half as wide and half as long as runway 22 and was unlit, Hersman said in a media briefing.

One possible explanation involved a temporary advisory to pilots warning of lighting outages on the longer runway that expired the day before the crash, aviation sources said.

However, the co-pilot of the ill-fated Comair flight flew into the airport on Friday night when the top half of the runway, where planes begin their takeoffs, had been dark. Aviation experts said he might have been unaware the advisory had been lifted and mistook the dark runway for the correct one.

NTSB has not been fully satisfied with efforts to reduce runway incidents. Even at heavily monitored airports, planes sometimes wind up on the wrong runway or taxiway.

"It's a major concern," said Carol Carmody, a former NTSB vice chairman.

At the airport, investigators drove stakes into the ground at the end of the runway and on the rolling hillsides beyond, where tarps protected evidence from a steady rain. Red directional signs clearly marked the two runways.

"One of the issues that we're certainly going to be looking at is the visibility and the ability for the crew to see," Hersman said. "And also the issue of whether or not air traffic control could see."

The airport tower was staffed at the time of the accident by a lone Federal Aviation Administration air traffic controller, who had been on duty all night.

Hersman said information was being gleaned from the plane's cockpit voice recorder and flight data recorder.

"Both were in good condition," she said. "We have about 32 minutes from the (cockpit voice recorder)."

Preflight preparations were normal and the aircraft, a CRJ-100 made by Montreal-based Bombardier, was deemed airworthy before takeoff, Hersman said.

"Finally, the takeoff roll began and the airplane continued to accelerate until the recording stops," she said.

Several teams are investigating different aspects of the crash, including visualizing the situation from the pilot's vantage point at the same time of day as the crash, she said.

"There are often issues that present themselves, whether weather or darkness or other things that could have obscured the view," she said.

The crash occurred one hour before sunrise, with the jet clipping several trees and leaving a long trail of debris.

The lone survivor among the 47 passengers and three crew members was the co-pilot, who was in critical condition.

The local coroner said the bodies had been removed from the burned-out fuselage and bodily fluids taken to perform toxicology tests.

"Toxicology reports are standard in accident investigations," Hersman said. "They generally look for alcohol and six illicit drugs."

Comair is a feeder carrier for Delta Air Lines. Both are restructuring in bankruptcy.