An Air Force jetliner packed with sophisticated missile-tracking equipment exploded in the sky over Frederick in rural Maryland yesterday morning, killing all 21 persons on board.

The plane, a Boeing EC135-N, was 50 minutes out of Wright-Patterson Air Force Base in Dayton, Ohio, when, for reasons as yet unknown, it blew apart at 10:51 a.m. at an altitude of 29,000 feet -- about 5 1/2 miles up. It was scheduled to fly east over the Atlantic Ocean before returning to its base at 3 p.m., according to Air Force spokesmen.

The plane was on a routine, unclassified training mission and carried no classified documents or equipment, a spokesman at Wright-Patterson said.

Witnesses told of seeing or hearing two or three thunderous explosions before pieces of the plane came to rest in the Stup family rye field near Walkersville just north of Frederick. Jimmy Stup, 19, who farms the 450 acres with his father, said he was in the kitchen of his home when he heard "a loud, awful rumble that sounded like it was going to take the roof of the house off. . . There seemed to be two simultaneous explosions and the plane just looked like a big ball of fire. There was no smoke."

Frank Harris, a traveling salesman from Emmitsburg, saw the plane come down as he was driving along near Rte. 194. "I heard an explosion and I saw a fireball immediately afterward," Harris said. "The explosion went, 'Kaboom, kaboom, kaboom.' There were three big explosions and then it went down out of my sight."

Stup and two farmhands jumped into a pickup truck and drove to the crash site. "There was computer parts and pieces of the plane everywhere," Stup said. Debris was scattered over a three-mile area, he estimated.

Stup said he walked through the field and saw about nine bodies, some slightly burned, all twisted grotesquely. "I didn't touch the bodies," he said. "I went back as soon as I could. It was awful, I'd only seen that on TV."

The Air Force withheld the names of the crew members until relatives were notified.Air Force officials said that all 21 on board were apparently stationed at Wright-Patterson. Their bodies were being taken last night to Bethesda Naval Hospital for identification.

There apparently were no fatalities or injuries on the ground, according to Maryland State Police.

There were conflicting reports throughout the day that classified documents were on the plane, and at one point a Frederick radio station broadcast an appeal that all documents be turned in. A spokesman said the Air Force had not requested the appeal.

The EC135-N, built by Boeing in 1960, was one of eight aircraft specially modified to acquire and record date from missiles and satellites, Air Force and Boeing officials said. The $50 million plane is nicknamed the "Droop Snoot" because of a unique, 10-foot-long nose cone/radar dome that carries a seven-foot-wide parabolic dish antenna, the largest ever flown.

That antenna, according to Air Force officials, is used to acquire computerized data from missiles and satellites when they are flying in areas difficult to cover with ground- or water-based tracking stations. The aircraft is not normally used for intelligence gathering, according to Air Force officials.

Air Force Maj. Arnold Michalke, an EC135-N pilot also stationed at Wright-Patterson, said in an interview last night that the addition of the snoot "does absolutely nothing" to change the feel of the aircraft for the pilot. "At some of the higher airspeeds you get a slight vibration on the nose," Michalke said. "It does create some noise." There are some speed restrictions, he said, but he knew of no difficulties with the snoot in the history of the modified plane.

The four-engine plane, he said, "Flies amazingly like a [Boeing] 707. If I stuck you in one blind-folded, you couldn't tell the difference."

The crash is being investigated by the Air Force, which dispatched a 100-man team from Andrews Air Force Base shortly after the accident and was flying in experts from Vandenburg Air Force Base, Calif.

There were no obvious early clues to the cause of the crash.

The Air Force said the plane was following its assigned course when the accident occurred. The plane's flight path was being monitored by the Federal Aviation Administration's Air Route Traffic Control Center in Leesburg, but all communication between the center and the aircraft was routine.

Angelo Bviselli, the center's chief, said the pilot and one of his controllers talked briefly when the plane entered the Leesburg Center's airspace, but had no further communications. The computerized radar system at Leesburg reported the plane's altitude every 20 seconds. Suddenly, the altitude readout disappeared from the radar screen, an unusual situation for a plane at high altitude.

Viselli said the controller "attempted to contact the aircraft, but received no answer." FAA offices in Leesburg, Martinsburg, W.Va., and Frederick began to receive calls from citizens who reported seeing or hearing explosions, Viselli said.

The Maryland State Police, among the authorities notified by the FAA, had a helicopter on the crash site within 15 minutes.

There was no possibility of a collision, Viselli said, because all other airplanes known to be in the area of the EC135-N were accounted for.

Viselli said that the weather was excellent for flying yesterday and that there had been no reports of clear-air turbulence, which results in a bumpy ride even though the skies are clear. Clear air turbulence has been a factor in or a cause of some aircraft accidents.

Aviation experts said yesterday that any attribution of cause this early in the investigation would be speculation. Nonetheless, they said, several things could cause an airplane to explode or break apart, including:

A malfunctioning control system that the crew could not correct.

A sudden illness that incapacitated the pilot. A stricken pilot might accidentally change a control setting.

A structural failure that would cause an engine or other large piece to separate from the airplane and strike a vital control surface, such as the wing or the tail.

A mistake by the pilot that would send the aircraft into a dive from which it could not recover. Such incidents, called "jet upsets," was an early problem with jetliners that were rectified by improved pilot techniques.

An explosion, the sources emphasized, could just as easily be the result of some other failure on the plane and not necessarily the cause. An explosion would undoubtedly involve the fuel system and could result from a ruptured fuel line or tank. The plane left Wright-Patterson with 140,000 pounds of jet fuel, or about 20,000 gallons, the Air Force said.

The EC135-N is a derivative aircraft of the military C135, which looks much like the commercial Boeing 707. However, Boeing officials said, the C-135 is somewhat smaller in length and wingspan than the 707 and has different engines. When the plane was being designed, Boeing officials called it the 717 -- the missing number in the now-famous Boeing 700 series of commercial airliners. More than 800 C135s have been delivered by Boeing to the Air Force.

Eight of the C135s built in 1960 were modified with the "Droop Snoot" in 1967 by McDonnell-Douglas at a plant in Tulsa. They were used initially to track the Saturn rockets in the Apollo space program and have been modified at least once since then. The plane that crashed is also known as an "ARIA," for Advanced Range International Aircraft.

The plane was assigned to the 4950 Test Wing of the Air Force Systems Command (AFSC). The command's headquarters is at Andrew Air Force Base.

A normal satellite-tracking mission would require a crew of 16, according to the Air Force, but for some specialized missions the crew might reach 23. The crew members would sit at electronic consoles in the cabin of the aircraft; the number of consoles would vary with the mission. Training missions are flown once or twice a week, Maj. Michalke said.

The investigation is the sole responsibility of the Air Force. However, FAA spokesman Fred Farar said, "We are concerned and will be in touch with the Air Force because we will want to know if there is any structural problem that could be transmitted to the civilian 707."

The FAA and an Air Force team at Wright-Patterson have been working jointly in recent years on the problems of what they call "geriatric airplanes," aircraft that are 20 or more years old and may be developing problems never anticipated when they were manufactured.

As a result of that study, the FAA just yesterday implemented a new regulation requiring beefed up inspections and maintenance programs, but that regulation would apply only to commercial planes. Such a program has already been developed for the Boeing 707.