MINA, S.D. –– Federal investigators returned today to the site where a jet carrying golf champion Payne Stewart nose-dived into a field, cautioning there would likely be no quick explanation about why the plane hurtled pilotless for 1,400 miles across the country before crashing.

Five others believed to be on board also were killed.

A six-member National Transportation Safety Board team walked through the crash site Monday evening, hours after the Learjet crashed. They did a cursory inspection of the wreckage, lit by generator-powered spotlights.

The wreckage is "fairly contained," NTSB Vice Chairman Robert Francis said today. "It looks the aircraft was pretty much vertical when it hit the ground. The ground is soft, and it went in fairly deep. It's going to be a challenge, with the wreckage and sorting out what's there."

The investigation will take some time, he said. "Our concern is making sure we do it properly than making sure that we do it fast."

Autopsies likely could reveal whether the plane's occupants suffered oxygen deprivation, Francis said.

"That's something we'll be working with the coroner on," he said. The bodies remained at the scene this morning.

Francis stressed that a sudden decompression of the aircraft is still just one possible theory of the cause of the accident.

Stewart, 42, had won 18 tournaments over his career, including two U.S. Open titles. He also was part of the team that helped the United States stage a historic comeback to beat Europe last month for the Ryder Cup.

Stewart and his wife had two children, Chelsea, 13, and Aaron, 10.

"He's an irreplaceable guy," fellow golfer Duffy Waldorf said. "I think of Payne Stewart and there's a guy that's going to be like Jack Nicklaus and Arnold Palmer, a guy you want around all those years."

Also killed were Stewart's agents, Robert Fraley and Van Ardan, and the two pilots, Michael Kling, 43, and Stephanie Bellegarrigue, 27. Nicklaus said he feared one of his golf course designers, Bruce Borland, 40, also died.

Borland's wife, Kate, said she contacted the private jet terminal Monday afternoon and spoke to an employee who confirmed her husband had intended to board the flight. And this morning, Francis said, "We believe there are six dead."

The jet was operated by Sunjet Aviation Inc. Company officials told The Wichita Eagle it had been inspected twice in the previous three days, including just before the doomed flight.

The 23-year-old, eight-passenger plane had logged more than 10,000 hours of flight time, but it had no history of serious mechanical problems, according to the Federal Aviation Administration.

The Lear 35 left Orlando, Fla., at 9:09 a.m. and was headed to Texas, where Stewart was to attend a meeting on a proposed golf course near Dallas and the Tour Championship in Houston.

Air traffic controllers soon lost contact with the chartered, twin-engine plane. It may have suddenly lost cabin pressure soon after taking off for Dallas, government officials said.

Planes that fly above 12,000 feet are pressurized, because the air at altitudes above that lacks enough oxygen to breathe comfortably. If a plane loses pressure, those aboard could slowly lose consciousness or, if an aircraft broke a door or window seal, perish in seconds from lack of oxygen.

Once reaching a cruising altitude, pilots often switch on the autopilot. If they pass out, the plane could continue on until it ran out of fuel.

Fighter jets sent after the Learjet followed it for much of its flight but were unable to help. The pilots drew close and noticed no structural damage but were unable to see into the Learjet because its windows were frosted over, indicating the temperature inside was well below freezing.

The FAA routed air traffic around the Learjet and kept planes from flying under it.

The plane, apparently on autopilot, cruised 1,400 miles straight up the nation's midsection, across half a dozen states. Authorities say the plane was "porpoising," fluctuating between 22,000 and 51,000 feet.

Stewart's Australian-born wife, Tracey, tried to reach her husband on his cellular phone while she followed the drama on television, her brother said.

"She was trying to ring him on his mobile and couldn't raise him. It's just really bad for my sister to be watching it on CNN, knowing that it was her husband on board," Mike Ferguson told the Australian Broadcasting Corp.

The plane presumably ran out of fuel some four hours after it took off, then crashed in the field in South Dakota.

"The plane in trouble started flip-flopping around and turning somersaults," said Ken Dunn of Mina, one of the first to arrive at the crash site. "When it came down, it came down."

Stewart, with his traditional knickers and tam-o'-shanter hat, was one of the most recognizable players in golf. Among his tournament wins were three major championships. In June, he won his second U.S. Open, prevailing over Phil Mickelson with an astonishing 15-foot putt on the final hole.

"This is a tremendous loss for the entire golfing community and all of sports," said Tim Finchem, commissioner of the PGA Tour. "He will always be remembered as a very special competitor and one who contributed enormously to the positive image of professional golf."

President Clinton said: "I am profoundly sorry for the loss of Payne Stewart, who has had such a remarkable career and impact on his sport and a remarkable resurgence in the last couple of years."