His life had long been wrapped in risk and mystery, whether it was undercover heroin work in Marseilles or cocaine in Mexico. He was, by all accounts, a most effective U.S. drug agent, involved in heavy investigations at the highest levels.

Now as he lies here on a life-support system, his legally declared death virtually imminent, one more mystery, a final and apparently fatal one, has been added to the extraordinary life of Sante Alesandro Bario:

Was he poisoned 10 days ago in his Bexar County jail cell while awaiting trial on charges that he took a bribe?

That is a question that follows logically from an earlier one:

was he in jail on bribery charges because he was framed?

Both questions lead inevitably to a third:

If so, by whom?

"There are lots of people who would be glad to get rid of him," one confidant said.

Whatever the answers, such is the dark ending to Bario's star-brilliant career. Whether he does die, as is feared, or whether he somehow survives with massive brain damage, he assuredly will never get the chance in court to clear himself, to challenge-whatever the verdict-the serious charges against him.

Those close to Bario assert that his defense in such trial inevitably would have involved disclosures of the inner, secret workings of the federal Drug Enforcement Administration's international drug investigations, and they say, the workings of other, unnamed intelligence-gathering agencies.

Not now, though. Not ever.

For 14 years, Bario made a name for himself in some of the biggest undercover investigations in the nation's drug-enforcement effort. The Drug Enforcement Administration will tell you that. Bario has his departmental awards to prove it.

A native Italian, Bario moved easily in European, American and Canadian mob circles, it is said. There were assignments in Boston and New Orleans.He was instrumental in locking up an assistant New York prosecutor in 1972 on bribery charges.

In that case, Bario, posing as a Las Vegas heavy facing a weapons charge, was wired to provide a secret recordings of evidence. The prosecutor did not press the charge and was arrested. The electronic method of investigation would come back six years later to spring like a trap on Bario.

In 1976, Bario was promoted partly because of his performance in infiltrating large drug-trafficking circles that embraced Belgium, France and Mexico.He was assistant regional director of DEA's office in Mexico City and was working on cocaine trafficking "involving more than one continent and major traffickers," one source said.

Then last Oct. 7 Bario went to San Antonio's Hilton El Palacio Hotel for a rendezvous with a DEA informant called "Claude Picault," an apparent cover name. "Claude Picault" was wired for sound by DEA, and Bario was arrested while heading for an elevator.

The three-count bribery indictment alleges that Bario Received $9,000 from "Claude Picault" in two installments, one in Chicago's O'Hare Airport and the other at El Palacio. The payment, it is alleged, was for overlooking five kilograms of cocaine in a 15-kilogram shipment. "Claude Picault" also may go by the name "Claude Hernandez."

At one pretrial court hearing, arresting investigators acknowledged that DEA agents often allow informants to possess narcotics and agents to receive payments during drug deals. There was no testimony that Bario was acting outside his official duties during his dealings with "Claude Picault."

He was held under $500,000 bond, and family and legal efforts to raise the money or lower the bond failed.

His wife Joanne (they have a 2-year old son thinks he was framed. She also testified at a bond hearing that Bario finally had cracked under the pressures of the netherworld of undercover investigation. He was under the care of a psychiatrist, who had prescribed medication.

Then came Saturday, Dec. 16. In a solitary cell at Bexar County Jail, Bario, despite statements to his wife that he could obtain only cookies after meal hours, was eating a peanut butter sandwich at 10:30 p.m. He suffered a seizure and choked on the sandwich, at some point going into convulsions.

Emergency medical technicians arrived. They found no heartbeat, no pulse, no breathing.

Suspecting a heart attack, they injected Lidocaine, a drug that can react fatally with Bario's prescribed medication, Elail.

At the hospital, a technician scanned a urine sample and found strychnine a finding consistent with the convulsions.

Bario it seemed had been poisoned. Not so says the Bexar County medical examiner. Using sophisticated computer analysis, the examiner. Dr. Ruben Santos said the substance that looked like strychnine was a broken down form of Elavil. At least in the urine sample that was sent to him there was no strychnine. And the strychnine-like substance he says, matched that in other patients receiveing Elavil.

He suggested that Bario perhaps had suffered a heart attack as a result of his medication.

Further tests revealed no heart problem.

The peanut butter sandwich was tested, but nothing was found.

At the same time, his doctors, who could not be reached told his lawyer, Gerald Goldstein, and his wife that they were treating Bario for strychnine poisoning or at least a toxin with the same effects as strychnine.

He lies in a San Antonio hospital, his body sedated into paralysis, but his brain waves showing convulsions.

With every passing moment that the machinery keeps Bario "alive," the chances of finding any toxin within him diminish. "By now it's too late to recover anything," the medical examiner said.

"Unless something physcial's there that we can recover, it will be lost."Blood and other samples have been refrigerated for future analysis. Perhaps only an autopsy, when death comes, will answer the questions.

"No matter how you look at this," Santos said, "it is going to be a mess. It will all be speculation." CAPTION: Picture, SANTE ALESANDRO BARIO . . . stricken in his cell