Air Force investigators today discounted the possiblility of sabotage and ruled out a collision in the air as possible causes of the crash here Wednesday of a missile-tracking plane. All 21 people on board were killed.

Brig. Gen. William T. Twinting, head of the board of investigation, said he has not confirmed that the plane exploded and came apart in the air, but said, "I have never seen a [crashed plane] so spread out that was the result of ground impact [alone]."

Twinting talked to reporters after they toured the wreckage of the Boeing EC135-N, parts of which were strewn across two grain fields divided by a railroad track two miles north of this rural Maryland Community.

The vertical stablizer -- the tail fin -- was located about 200 yards south and west of the main section of the fuselage, which fell in a stand of trees that were defoliated in the fire that accompanied the crash. The cockpit buried itself in the ground 120 feet north of the main fuselage section and had to be dug out.

The two center engines of the aircraft were located on the railroad tracks at least 100 yards north of the vertical stablizer. The two outboard engines were another 100 yards east and separated by at least 100 yards. Pieces of wing and fuselage, and struts and stringers from the fuselage and wing boxes, were scattered throughout the area, as were chunks of wiring and pieces of hydraulic and fuel lines.

A charred flight jacket bearing the shoulder patch of the 4950th Test Wing from Wright-Patterson Air Force Base, where the flight originated, was piled casually against a flight engineers' panel.

Pieces of a radar dome, a 10-foot-long protrusion on the nose of the aircraft that gives it a unique appearance and the nickname "Droop Snoot," were located throughout the crash site, Twinting said. The dome, he said, was of more fragile material than the rest of the craft.

With the exception of a midair collision, Twinting said, "I haven't ruled out anything." In answer to a direct question, he said, "I have no evidence whatsoever of sabotage." Rumors of sabotage have received widespread attention from the media, principally because of the presence of FBI agents on the scene. However, FBI agents are always on the scene of major plane crashes, military or civilian.

All major parts of the craft were located within a square-mile area, Twinting said. That kind of disperasion, according to aviation experts not connected with the investigation, is consistent with a breakup of the plane in the air.

"If just one major structural part of an airplane fails for some reason, all of the other parts become overloaded," one experts said. "The loads on the rest of the plane are so severe there is no way they can withstand them. There is no predictable pattern from that point on how the aircraft will break up."

The explosions heard by witnesses doubtless involved the fuel on the aircraft, but could have occurred after the plane began breaking up rather than being the cause. Air Force officials said the plane was not carrying ordnance or explosives.

The plane was on a routine, unclassified training mission. It was last seen on the radar at the air traffic control center in Leesburg, Va., at 10:51 a.m. Wednesday. At that time the plane was reporting an altitude of 29,000 feet, and all air traffic control communications had been normal.

Suddenly the plane dropped from the radar screen and controllers were unable to regain contact.

James Kinsey, who lives near Thurmont, Md., told reporters yesterday that he had been fishing about 5 miles north of the crash site. It was rainy and foggy. "I heard the sound of a plane coming from my right to my left; it sounded low. It was roaring when it was over my head, and suddenly the sound stopped. I never heard an explosion or a crash." Kinsey's report would indicate that the engines of the craft stopped functioning for some reason, according to aviation experts.

The engines, like other parts of the plane, will be taken to Andrews Air Force Base next week for further study by Twinting's experts. Twinting said he hoped to be clear of the crash site by midweek and to complete the investiagtion within three weeks.