MINA, S.D.--A Dallas-bound Learjet carrying U.S. Open golf champion Payne Stewart knifed nose-first into a grassy field southwest of Mina, S.D., on Monday after a ghostly ramble across America with no one at the controls.

All five people listed on the flight manifest -- Mr. Stewart; his agents, Robert Fraley and Van Ardan; and pilots Michael Kling and Stephanie Bellegarrigue -- were killed, officials said.

A sixth person, a golf course designer from North Palm Beach, was also believed to have died on the plane, according to his employer, Nicklaus Design.

Jack Nicklaus said Monday that Bruce Borland, 40, was flying to Texas because he wanted to design a course with Mr. Stewart under the Nicklaus Design banner.

Authorities could not confirm that Mr. Borland was on the plane, and officials at the crash site said they could not tell exactly how many people had been killed.

No one on the ground was hurt.

Flight controllers frantically tried to contact the Lear 35 by radio, and a series of military jets scrambled to chase the plane as it streaked along, guided by its auto-pilot system until the fuel supply finally ran out.

Capt. Kevin Bakke of the South Dakota Highway Patrol said he watched for three or four minutes as the plane sped across the sky, followed by two military jets.

Then a deputy thought he saw a parachute, but it was the plane plummeting toward the ground, Capt. Bakke said.

"It fell out of the sky," he said. "There was no glide to it."

Investigators said everything they know so far about the flight is consistent with hypoxia, the condition caused by a lack of oxygen that leads to confusion and ultimately to death.

"We figure these guys were dead for most of the trip," one source said.

The last contact with the plane came near Gainesville, Fla., when one of the pilots spoke with flight controllers in Jacksonville, according to a Federal Aviation Administration spokesman.

But there was no hint of trouble until the Lear failed to turn west over Florida's panhandle for its flight into Dallas Love Field.

Mr. Stewart, 42, decided to stop in Dallas to meet with developers building a golf course near the Dallas North Tollway in Frisco that would carry Mr. Stewart's name.

Mr. Stewart, who played golf at Southern Methodist University before beginning his successful professional career in 1979, was en route from his home in Orlando to Houston, site of this week's Tour Championship. It is the Professional Golfers Association's final tournament of the year for its top 30 money winners.

"It is difficult to express our sense of shock and sadness over the death of Payne Stewart ," said PGA Tour Commissioner Tim Finchem in a statement issued from PGA headquarters in Ponte Vedra Beach, Fla.

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

Fellow golfer Duffy Waldorf, in Houston practicing for the Tour Championship, told KTRH-AM that he considered turning around and going home when he heard the news.

"He's an irreplaceable guy, not just for his playing record," Mr. Waldorf said.

Mr. Stewart was among the best-known players on the professional tour, in large part because of his distinctive sartorial style. In a sea of baggy khakis, Mr. Stewart was singularly elegant in his trademark knickers and snappy English-style caps.

This had been a particularly successful year for Mr. Stewart, who suffered a long, difficult drought through much of the '90s before winning the AT&T Pebble Beach National Pro-Am and then his second U.S. Open championship, sinking a 15-foot putt on the final hole to beat Phil Mickelson.

"I'm proud of the fact that my faith in God is so much stronger and I'm so much more at peace with myself than I've ever been in my life," Mr. Stewart said after the win. "Where I was with my faith last year and where I am now is leaps and bounds."

The Rev. Jim Henry, retired pastor for First Baptist Church of Orlando who used to minister to the Stewart family, was one of those who gathered Monday outside the family's home.

"He was a wonderful Christian who had Christ in his life and somehow in his death," Mr. Henry said. "That brought a great sense of peace to his family in a difficult and tragic time."

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

The Learjet that carried Mr. Stewart and his party was operated and managed by Sunjet Aviation of Stanford, Fla. It was manufactured in 1976 and had made roughly 7,500 flights, company officials said.

All required maintenance was current, the officials said.

Determining just what went wrong could be very difficult because the plane was completely destroyed at impact, investigators said.

The Learjet 35, a light twin-turbofan business jet that can carry eight passengers and a crew of two, is equipped with an oxygen system for emergency use, with masks for the crew and each passenger.

It also has a depressurization warning system.

Investigators from the National Transportation Safety Board will be looking for anything that might explain how the plane could have lost oxygen without triggering the alarm or anyone noticing.

If the plane rapidly lost pressurization for some reason, all the occupants would have had masks to wear until the plane reached a lower altitude.

But if the oxygen system malfunctioned, the passengers could have been slowly starved of oxygen, causing hypoxia. That condition provides only subtle signs, beginning with a feeling of well-being or giddiness. Within a few minutes, confusion sets in, followed by unconsciousness and death.

Such losses of cabin pressure are rare in private aircraft, experts said, but there have been similar cases.

In 1980, newly named Louisiana State University football coach Robert "Bo" Rein died when a private plane veered 1,000 miles off course at extremely high altitudes and then plunged into the Atlantic Ocean.

The flight was supposed to go from Shreveport to Baton Rouge. The plane crashed off the coast of Virginia.

And in 1988, a Learjet bound for Addison Airport from Memphis, Tenn., overflew its target by hundreds of miles, crashing in Mexico.

In both crashes, officials blamed hypoxia.

Mina resident Lesley Braun, who lives about two miles from the crash site, said her husband saw the plane carrying Mr. Stewart spiraling down.

"The plane had pretty much nosed straight into the ground," Mrs. Braun said. "There's not a lot of debris spread out a long ways."

The force of the crash compressed the plane to roughly the size of a large pickup truck. The only part clearly recognizable as a piece of aircraft was the tail cone, resting atop the wreckage.

"If you didn't know it was a crash site, you'd think you stumbled across a trash pile," said Capt. Bakke of the highway patrol.

The plane crashed in a field that was once wetlands, and Capt. Bakke said it's possible parts of the wreckage are buried 15 to 20 feet deep.

At a news conference Monday, President Clinton said he was profoundly sorry about the death of Mr. Stewart and the others who died in the crash.

"It's a very sad day," he said.

"I am very grateful for the work the FAA did and for the two Air Force pilots and the others in the Air Force that monitored this plane and made every effort to try to make contact with it.

"They did everything that could humanly be done, and they were looking out for the safety of everyone involved."

There was some speculation Monday that the military jets were prepared to shoot down the Lear if it threatened to crash in a heavily populated area. But officials at the Pentagon strongly denied that possibility.

Shooting down the plane "was never an option," Air Force spokesman Capt. Joe Della Vedova said. "I don't know where that came from."

Instead, according to an Air Force timeline, a series of military planes provided an emergency escort to the stricken Lear, beginning with a pair of F-16 Falcons from the Air National Guard at Tyndall Air Force Base, Fla., about 20 minutes after ground controllers lost contact.

An F-16 and an A-10 Warthog attack plane from Eglin Air Force Base, Fla., took up the chase a few minutes later and were trailing the Lear when it climbed abruptly from 39,000 to 44,000 feet at 9:52 a.m. CDT.

Fifteen minutes later, the F-16 intercepted the Lear, the pilot reporting no movement in the cockpit.

At 10:44 a.m., the fighters from Eglin diverted to St. Louis for fuel. Fifteen minutes later, four Air National Guard F-16s from Tulsa, Okla., took up the chase, accompanied by a KC-135 refueling tanker.

F-16s from Fargo, N.D., later scrambled to intercept the Lear jet , too. At noon Dallas time, the Fargo F-16s reported that the windows of the jet were fogged with ice and there was no evidence anyone was piloting the plane.

At 12:14, the Lear jet began to spiral. It crashed about six minutes later.

Staff writers Terri Langford, Kathy Lewis and Richard Whittle and the Associated Press contributed to this report.