The windows were frosted up, suggesting a massive and immediate loss of pressure on the plane. It flew on like a ghost plane over central US states for several hours.

It appears the five passengers were incapacitated early in the flight, although investigators are searching for the 'black box' recorder for evidence.

Lear cabins are pressurised

The plane, a Lear 35 built in 1976, took off from Orlando, Florida, at about 0920 local time (1420 BST).

Mr Stewart had been expected in Houston for practice rounds in advance of the PGA Tour Championship.

But the plane veered off course and lost radio contact with the ground soon after it took off - the last word from the flight coming when it was over Gainesville, Florida.

When the crew later failed to respond to repeated inquiries from air traffic controllers an Air Force fighter jet from Tyndall, Florida, was diverted from a routine training flight to check the Learjet.

Two fighters from Eglin Air Force Base, Florida, then took over to follow it and they later handed off the monitoring to two Air National Guard F-16s from Tulsa, Oklahoma.

Investigators scour the crash site

About four hours after takeoff, the plane crashed in a grassy field 3km west of Mina, South Dakota.

Planes that fly above 3,600m (12,000ft) are normally pressurised, because passengers would have difficulty breathing the thin air above that altitude.

If there is a pressurisation problem, those aboard the aircraft could slowly lose consciousness or, if an aircraft broke a door or window seal, perish in seconds from hypoxia, or oxygen deficiency.

Once reaching a cruise altitude, pilots often switch on the autopilot. If they passed out, the plane would cruise until it ran out of fuel.

British aviation expert David Learmount, of Flight International, said: "We understand the Learjet was at 43,000ft which is well above what most airliners fly at.