LEXINGTON, Ky. — Comair Flight 5191 was on a runway too short for its size and weight in the seconds before it crashed early yesterday and burst into flames, killing 49 people and leaving the lone survivor — a co-pilot — in critical condition, federal investigators said.

Preliminary flight data from the plane’s black boxes and the damage at the scene indicate the plane took off from the shortest runway at Lexington’s Blue Grass Airport, a National Transportation Safety Board member, Debbie Hersman said.

The 3,500-foot-long strip, unlit and barely half the length of the airport’s main runway, is not intended for commercial flights.

It was not immediately clear how the plane ended up there in the pre-dawn dark.

“We will be looking into performance data. We will be looking at the weight of the aircraft. We will be looking at speeds. We will pull all that information off,” Ms. Hersman said.

Aviation experts said the twin-engine CRJ-200 regional jet would have needed 4,500 feet to fully get off the ground.

The Atlanta-bound plane smashed through a perimeter fence and crashed in a field less than mile from the end of that runway at about 6:07 a.m. Aerial images of the crash site in the rolling hills of Kentucky’s horse country showed trees damaged at the end of the short runway and the nose of the plane almost parallel to the small strip.

When rescuers reached it, the plane was largely intact but in flames. A police officer burned his arms dragging the only survivor from the cracked cockpit.

The flames kept rescuers from reaching anyone else aboard — a newlywed couple starting their honeymoon, a Florida man who had caught an early flight home to be with his children, and a University of Kentucky official among them.

“They were taking off, so I’m sure they had a lot of fuel on board,” the Fayette County coroner, Gary Ginn, said. “Most of the injuries are going to be due to fire-related deaths.”

An FAA spokeswoman, Laura Brown, said the agency had no indication that terrorism was involved in any way in what was the country’s worst domestic plane crash in five years.

It’s rare for a plane to get on the wrong runway, but “sometimes with the intersecting runways, pilots go down the wrong one,” a Saint Louis University aerospace professor emeritus, Paul Czysz, said.

The worst such crash came on October 31, 2000, when a Los Angeles-bound Singapore Airlines jumbo jet mistakenly went down a runway at Taiwan’s Chiang Kai-Shek International Airport that had been closed for repairs because of a recent typhoon. The resulting collision with construction equipment killed 83 people on board.

At the Lexington airport, the shorter runway veers off at a V from the main strip.

Comair’s president, Don Bornhorst, said the plane’s maintenance was up to date and that its three-member flight crew was experienced and had been flying that airplane for some time.

“We are absolutely, totally committed to doing everything humanly possible to determine the cause of this accident,” Mr. Bornhorst said. “One of the most damaging things that can happen to an investigation of this magnitude is for speculation or for us to guess at what may be happening.”

Most of the passengers aboard the flight had planned to connect to other flights in Atlanta and did not have family waiting for them, the Reverend Harold Boyce, a volunteer chaplain at Atlanta’s Hartsfield-Jackson airport, said.

One woman was there expecting her sister. The two had planned to fly together to catch an Alaskan cruise, Mr. Boyce said.

“Naturally, she was very sad,” Mr. Boyce said. “She was handling it. She was in tears.”

The only survivor of the crash was identified as first officer James Polehinke, who was in critical condition after surgery at the University of Kentucky hospital.

The other crew members were Captain Jeffrey Clay, who was hired by Erlanger, Ky.-based Comair in 1999, and a flight attendant, Kelly Heyer, hired in 2004. Mr. Polehinke has been with Comair since 2002.

The plane had undergone routine maintenance as recently as Saturday and had 14,500 flight hours,”consistent with aircraft of that age,” Mr. Bornhorst said.

Investigators from the FAA and NTSB were at the scene, and Mr. Bornhorst said the airline was working to contact relatives of the passengers.

A White House spokeswoman, Dana Perino, said President Bush, who is spending a long weekend at his family’s summer home on the Maine coast, was being briefed on the crash.

“The president was deeply saddened by the news of the plane crash in Kentucky today,” she said. “His sympathies are with the many families of the victims of this tragedy.”

Among those killed were a newlywed couple starting their honeymoon. Jon Hooker, a former minor-league baseball player, had just married Scarlett Parlsey the night before the crash in a fairy tale wedding ceremony complete with a horse-drawn carriage and 300 friends.

“It’s so tragic because he was so happy last night,” Keith Madison, who coached Hooker’s baseball team at the University of Kentucky and attended the wedding, said. “It’s just an incredible turn of events. It’s really painful.”

The crash marks the end of what has been called the “safest period in aviation history” in America. No major crash has occurred since November 12, 2001, when American Airlines Flight 587 plunged into a residential neighborhood in Queens, N.Y., killing 265 people, including five on the ground.