A commuter plane that crashed while approaching an airport this week lacked an updated system that warns pilots who fly too low, equipment that will be required next year, investigators said.

Instead, the Corporate Airlines (search) twin-engine turboprop had an earlier version of the terrain warning system that met current regulations, National Transportation Safety Board (search) spokesman Keith Holloway said.

The 19-seat BAe Jetstream 32 (search) was en route from St. Louis to Kirksville Regional Airport (search) in northeastern Missouri when it crashed Tuesday, killing 13 of the 15 people aboard. It was the worst civilian air crash in the United States since 21 people were killed Jan. 8, 2003, in a crash in Charlotte, N.C.

The role of the cockpit warning system was "just one of many aspects of the investigation," Holloway said Friday.

Information from the plane's voice and data recorders and traffic control tapes suggest the plane's approach to the airport was routine, said Carol Carmody, who is leading the team of NTSB investigators.

The data show the plane descending steadily, then climbing slightly in the last four seconds, she said. Investigators were still determining exactly how far above the ground the plane was at that point.

The flight was the sixth of the day for the crew, who had been on duty nearly 15 hours that day, within FAA-approved limits, investigators said.

The Federal Aviation Administration (search) ordered the terrain-warning system installed by next March 29 in commercial planes with at least six seats. It displays the surrounding area on a cockpit screen. If the plane flies dangerously near the ground or an obstacle, such as a building, a computer-generated voice calls out a warning.

The older system shuts off automatically during landing.

A spokesman for Smyrna, Tenn.-based Corporate Airlines, which operated the flight under contract with American Airlines' commuter service, said Friday the new system had been installed in two of the 11 planes in service.

"They were complying with the regulations in place. It's just sad that they hadn't gotten around to doing it," said John David, deputy chairman of the National Safety Committee for American Airline's pilots union.

Carmody said the captain can be heard on the flight recording spotting the airport. "Thirteen seconds later there was the sound of an impact on the recording, and three seconds later the recording ended," she said.

"There was no change in direction, speed or heading. There was no emergency call from the aircraft," she said.

She declined to speculate on whether the weather may have played a role in the crash. Skies were overcast and misting, with thunderstorms in the area.

The aircraft's maintenance records over the last 30 days were "very unremarkable," she said.

The two survivors, Dr. John Krogh, 69, and his assistant, Wendy Bonham, 44, remained hospitalized. Carmody called their survival "remarkable."

Most of the passengers were heading to a medical conference in Kirksville.