Postal Service investigators first became suspicious when an Eastern Airlines employe found a stowaway inside Flight 82's cargo bin, from which six pouches of registered mail containing $350,000 worth of rare coins, gold bullion, narcotics and jewelry had vanished mysteriously.

The stowaway had been discovered by accident as an airline crew was unloading a huge black trunk marked "Fragile: Musical Instrument." As the trunk was pushed from the bin, it opened and a pair of feet popped out. They were attached to a 6-foot-1, 190-pound man who said he had wanted to see if he could fly undetected from Los Angeles to Atlanta.

Investigators didn't believe him. He was their prime suspect in the mail theft. But what had he done with the loot?

Over the next few hours, postal inspectors found out, unraveling what became known as the "Jack-in-the-box" case, "a near perfect crime," in the words of a U.S. attorney. The Postal Service now cites the case as an example of how sophisticated mail thefts have become.

In 1981 the Postal Service reported 202,533 complaints about missing or stolen mail, up from 155,000 the year before. Postal thefts, especially thefts of food stamps, are on the increase, said W. G. Moore, head of the Postal Service's criminal investigations division.

"Postal thefts mirror the economy. When times are tough, thefts increase," Moore said.

Not all of the complaints involve theft, he said. Sometimes mail is missing because it was delivered to the wrong address. Other times it is ruined or destroyed accidentally by the Post Office.

For example, the Air Florida jet that crashed here in January was carrying two 40-pound pouches of mail, Moore said, and much of it was ruined. Many times, he said, there is no proof that mail reported stolen was.

Even so, copies of each complaint are sent to Moore's division for review. The Postal Service has 1,900 inspectors, who are trained at the agency's law enforcement school in Potomac.

The inspectors don't investigate every complaint they receive. Instead, they search for "attractive losses," either large numbers of complaints reported in one area or complaints involving large sums of money.

When they find such a case, investigators try to determine whether postal employes were involved. If some of the Postal Service's 666,000 employes are suspects, investigators usually will test them by arranging for fictitious money orders or valuable securities to pass through their hands. If employes aren't involved, investigators use the same sort of techniques used by other police.

Last year 259 employes were convicted of embezzling $547,865. Auditors also found 422 instances, totaling $241,741, of shortages in Post Office accounts. Another 2,442 were convicted of mail theft last year, according to Moore. He said 90 percent of the criminal cases filed by his inspectors ended in convictions.

Postal inspectors also investigate prohibited mailings, mail fraud and mail-order complaints. Last year they investigated 237 cases of mailed pornography, including 214 cases of child pornography; 300 cases of narcotics distribution and 113 incidents of mail bombs, including five related to terrorists.

Moore said 1,046 persons were convicted of mail fraud last year. The Post Office also received 45,030 complaints about mail-order firms. About 86 percent of the complaints were resolved, Moore said.

The criminal division, with 17 regional offices and an annual budget of $79 million, also investigates violent crimes against on-duty postal employes. Last year it investigated 213 burglaries and 127 armed robberies.

Besides investigators, the criminal division employs security guards, who police major Post Offices, and operates six criminal laboratories that specialize in handwriting analysis and forgery.

Since 1980, inspectors said, sophisticated mail crimes have increased. Recent investigations have included a major fraud case involving members of the Chicago police department and a $2 million diamond heist by a rock band, The Whispers.

The "Jack-in-the-box" case, however, is considered the most clever. It happened May 14, 1980, when William F. DeLucia's feet popped out of the black trunk.

Inspectors knew that the only reason DeLucia was discovered was because a clasp on the trunk had broken. So they reasoned that he must have had an accomplice waiting to claim the trunk. He did, but neither man provided any clues as to where the mail had been hidden.

Inspectors decided to take a long shot. The man waiting to retrieve DeLucia's trunk had been carrying a flight bag that he had left at the baggage claim area. Inspectors decided to see if anyone had picked up that bag. They spotted it with a third man, who had several other suitcases that had been taken off Flight 82. When they searched his luggage they found the missing mail.

Investigators theorized that after the plane was airborne DeLucia had climbed out of the trunk though a secret door. He then emptied the pouches into the suitcases that had been checked onto the airplane empty.

Prosecutors, however, still could not prove that DeLucia had opened the pouches. But they got the missing piece of evidence when technicians at the Postal Service's central lab here proved that a pair of pliers found on DeLucia had been used to cut through tin seals on the pouches. A jury convicted all three men of mail theft.