On the morning of Sept. 11, an elderly Knoxville, Tenn., man looked out his window and discovered a dead sky diver in his gravel driveway.

At the scene, police discovered the body of Andrew (Drew) Carter Thornton II, 40, clad in combat fatigues, a bulletproof vest and infrared night goggles. Thornton was carrying two handguns, a stiletto, the keys to an airplane, a money belt with $5,000 and six krugerrands, survival food, and 34 kilos of pure cocaine.

Thornton's death provided the first major clue in a complex cocaine-smuggling investigation that now involves the FBI, the Drug Enforcement Administration and police in Tennessee and Georgia.

Events in the case have focused attention on yet another advance in the endlessly resourceful illegal drug industry: the "vertical transport" of drugs using planes and parachutists.

Sky diving is a sport that attracts people who like excitement and risks, many of them highly trained former military jumpers and pilots who are ideally suited to the danger and challenge of drug smuggling. Acquaintances described Thornton as a "Rambo" type, referring to the current motion picture.

Most parachuting centers operate out of small, remote airports. The aircraft usually fly sky divers on weekends with plenty of down time at night and during the week.

Investigators learned that Thornton was a former Army paratrooper in the prestigious 82nd Airborne Division, that he worked for nearly a decade as a narcotics and intelligence officer for the Lexington, Ky., Police Department, and that he was a law school graduate, a convicted drug smuggler and the son of a wealthy Lexington horse-breeding family. He was on probation for the drug charge when he died.

Not long after his body was found, his plane, a twin-engine Cessna 404 that apparently had been left on automatic pilot, was found crashed into a ridge in a remote area of Nantahala National Forest in North Carolina. The plane was described by a friend of Thornton's as a "smuggler's dream" because of long-range fuel tanks that made it ideal for a trip from Colombia to the United States.

Three days later, two U.S. Forest Service officers discovered three duffel bags with markings identical to those found on Thornton filled with 99 more kilos of cocaine. They were attached to a cargo parachute found in a mountainous, remote area of the Chattahoochee National Forest on the flight line between Atlanta and Knoxville.

As if the mystery were not complicated enough already, an even deadlier and more puzzling event occurred in the sky near Atlanta Sept. 29: David (Cowboy) Williams, 35, a wealthy Atlanta real estate developer and former Vietnam helicopter pilot, died along with the pilot and 15 other sky divers when his plane crashed in a field near the West Wind Sport Parachute Club about 50 miles south of Atlanta.

Williams was known as a friend and sky-diving buddy of Thornton's, and the FBI is investigating the possibility of sabotage.

The National Transportation Safety Board later announced that a lab report indicated sugar and substantial amounts of water were in the fuel tanks of the $750,000 Cessna Caravan, a single-engine turboprop popular with jumpers because of its short runway requirements and fast rate of climb.

NTSB spokesman Ira Furman said it has not been determined whether the sugar actually reached the engine and clogged it. The investigation, which the NTSB said includes an initial indication that the plane may have been over weight limits, is continuing.

No link has been revealed between Thornton's plunge from the sky and the crash involving his friend Williams, or between the fate of the Cessna Caravan and drugs. But the events, so close in chronology and geography, could not help but become intertwined in speculation, and a possible connection is being studied by authorities.

One longtime official in the sport, who asked not to be identified, said, "I have't had anybody admit this to me, but I can't imagine that sky divers haven't been used for the vertical transport of drugs for years. What do you do with the drop zone airplane between Monday and Friday? And lots of sky divers like to live in the fast lane."

In the past 10 years, the DEA has recorded 57 drug-smuggling incidents involving parachutes, with the peak years being 1983 and 1984, according to spokesman Bill Deac.

So far, the investigations into the deaths of Thornton and Williams have raised more questions than they have answered.

Inspector Jimmy Davis, who supervises the drug enforcement section of the Georgia Bureau of Investigation, said he had a report about a week before Thornton's death that drugs were being parachuted into the state, but had no named suspects.

Enforcement sources say they believe that Thornton's flight may have been carrying 400 kilograms (or 880 pounds) of cocaine, meaning that authorities have accounted for only one-third of the load. The full load would have a wholesale value of about $16 million, with a street value of five to 10 times that amount, according to drug enforcement sources.

Law enforcement officials say they believe that Thornton, the parachutist whose body was found in Knoxville, had help waiting on the ground and that a second jumper may have left Thornton's plane, possibly with some of the cocaine. Local press reports have quoted unnamed investigators as identifying that jumper as Williams. But other agents working on the case say that is just speculation, that there is not enough evidence yet to identify the second jumper.

Investigators say they are not sure whether Thornton planned to jump all along or whether he bailed out after becoming suspicious that he was being trailed. The DEA sometimes trails drug planes by concealing a second transponder, which sends an electronic identifying signal, or by replacing the plane's own transponder.

Shortly after Thornton's death, investigators found a bag of Thornton's belongings -- including maps of Jamaica and a flight manual bearing the numbers of his plane -- in a pond near the West Wind drop zone where Williams operated his sky-diving plane. It was not clear whether the bag was brought there and dumped or whether it might have blown out of the plane when the door was opened in flight.

A flight from the drop zone over the site of the Chattahoochee Forest drop and on to Knoxville would be in a straight line almost due north and would have taken about 45 minutes in Thornton's plane.

Meanwhile, investigators led by the FBI are trying to determine who might have tried to sabotage Williams' plane, leading to the death of 16 other people. Atlanta FBI Agent David Kelsey refused comment on news reports saying the sabotage was committed by Colombians who think their cocaine was stolen by Thornton and Williams. Another federal agent, however, said sugar in the fuel tanks was more subtle than the usual Colombian revenge, which often comes in the form of a machine-gun attack.

Agents say, however, that they believe Colombian drug smugglers probably are looking for some of the major players who may have been involved in providing the cocaine that plunged from the sky with Thornton in an attempt to obtain payment for the missing drugs.

Two troublesome incidents involving fuel contamination preceded the crash of Williams' plane.

In mid-September, just after Thornton's death, Williams had flown the plane to a drop zone in Xenia, Ohio, where, according to jumpers there, an unusual fuel contamination problem, cause unknown, was discovered. A jumper at the airport said in a telephone interview that Williams then drained the tanks, filled them with good fuel and flew the plane without incident.

On the Friday before his crash, acquaintances said a Federal Aviation Administration inspector spotted fuel contamination in his plane while it was tied down at the Fulton County Airport on the northwest outskirts of Atlanta.

An experienced pilot who knew Williams said he did not drain the tanks and take the FAA inspector for a check ride, as he had been asked to do. Instead, Williams put in two cans of Prist, a fuel additive that retards fungus growth, a common contaminant in jet fuel, and flew the plane to the Dekalb-Peachtree Airport in northeast Atlanta, where it normally was based.

On Saturday, he flew the plane to a grass strip at the drop zone, flew several flights for the jumpers and then took it back to Dekalb-Peachtree for the night.

The next morning he flew the Caravan back to the drop zone and boarded with the sky divers while another pilot took the controls.

Witnesses have told investigators the plane took more runway space than usual before becoming airborne. No one is sure exactly how high it climbed, but estimates range from several hundred to 1,000 feet. At that point it stalled, the nose swung down toward the ground and it plowed into a field about a half-mile from the drop zone. Everyone was killed instantly.