An Air Force spokesman said today that investigators are still baffled as to why a missile-tracking plane exploded and crashed two miles north of here May 5, killing all 21 people on board.

Investigators have confirmed, Maj. William Campbell said, that the plane "obviously came apart before impact" with the ground -- "but not at 29,000 feet."

What remains unknown is what caused the plane suddenly to drop before breaking apart, exploding, and falling in a grain field.The plane, a four-engine Boeing EC135-N, was last recorded on radar at 29,000 feet.

Evidence available at the crash site suggests that the tail assembly fell some distance from the rest of the aircraft. The loss of the tail would make the plane inoperable and could cause the rest of the aircraft to break apart.

If the tail was the first major structure to leave the aircraft as it began breaking apart, however, that would not necessarily identify the cause of the accident. Something else catastrophic could have caused strains the tail could not withstand.

There is only one known clue so far that something like that may have happened: the sudden disappearance of the plane's altitude readout from the radar screen. Altitude information is generated by the aircraft and broadcast to the the radar screen. The fact that it was not available indicates some kind of power or equipment failure on the aircraft.

Campbell said additionally that none of the radio communication between the pilot and his Air Force command post has been in any way helpful to investigators. The pilot had the ability to communicate on Air Force channels as well as with air traffic control.

Investigators are hampered by the lack of devices that would have been carried aboard a commercial airplane -- a cockpit voice recorder, which records the conversation of the crew, and a flight data recorder, which provides a record of such items as engine settings and control positions.

Those instruments, Campbill said, are not required on Air Force planes. The Federal Aviation Administration requires them on commercial jets specifically to aid in the investigation of accidents or incidents. Col. Russel G. Westcott Jr., the deputy chief of the Air Force Board of investigation, bemoaned the lack of similar equipment on the Air Force jet during a brief interview last Saturday. "The only thing better," he said, "would be an interview with the pilot."

To date, investigators have arranged the pieces of each major structure -- the wings here, the fuselage there, the tail somewhere else. The four engines, Campbell said, all separated from the aircraft before it hit the ground. All engines have been inspected, he said, and there is nothing to indicate an abnormality. At the time of the crash, experts have estimated, the aircraft was carrying about 70,000 pounds of fuel, or about 10,000 gallons. t

The investigating team plans to finish work on the crash site early next week, then move to Andrews Air Force Base to evaluate what it has learned and to write a final report.