Kansas City, Mo. ? The cause of a commuter plane crash that killed 13 people remained a mystery, but investigators said the plane lacked an updated system that warns pilots when they fly too low.

The Corporate Airlines twin-engine turboprop had an earlier version of the terrain warning system that met current regulations, National Transportation Safety Board spokesman Keith Holloway said. The updated equipment will be required next year.

The 19-seat Jetstream 32 was en route from St. Louis to Kirksville Regional Airport in northeastern Missouri when it crashed Tuesday, killing 13 of the 15 people aboard.

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

“Nobody knows what caused it,” said Brannan Atkinson, a spokesman for Smyrna, Tenn.-based Corporate Airlines. “So nobody knows what could have prevented it.”

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 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.

Most large airliners already have been equipped with the enhanced technology, which is “one of the finest safety technologies that has ever come along,” said Capt. John Cox, a veteran airline pilot and safety chairman of the Air Line Pilots Assn.

The system “knows where runways are, it knows that the airplane’s not near a runway and it knows how high the ground is as well,” Cox said.

That information can be crucial when pilots are flying in the dark with perhaps a single light visible, he said.

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 Airlines’ 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.

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

The aircraft’s maintenance records over the past 30 days were “very unremarkable,” she said.

The two survivors, Dr. John Krogh, 69, and his assistant, Wendy Bonham, 44, remained hospitalized in fair condition early Saturday. Carmody called their survival “remarkable.”