Alfonso Valdivieso moves about Bogota with all the security normally accorded a head of state: a caravan of armor-plated cars, a posse of 15 heavily armed bodyguards. The picture window in his fifth-floor office offers a view that is distorted by the 2-in.-thick armored glass. Supposedly, the window can withstand a direct hit by a bazooka rocket.

The precautions are not paranoid. As Colombia's Prosecutor-General, Valdivieso is one of the most powerful and feared men in his very violent country. It follows that he is perhaps its most endangered citizen. Since his appointment in 1994, Valdivieso has systematically exposed the criminal-political conspiracy that sheltered 80% of the world's cocaine trade and earned his nation infamy as a "narco-democracy." Willing to take risks no predecessor in law enforcement dared, he has built cases against six of Colombia's drug-trafficking kingpins in recent months.

The investigations have Colombians applauding Valdivieso's audacity, yet his biggest achievement may still be ahead of him. Last month he jailed the former Defense Minister, Fernando Botero, for allegedly soliciting drug money for the 1994 campaign of President Ernesto Samper. Many observers believe the gathering scandal may ultimately force the President's resignation.

With so many powerful enemies--he is also investigating more than a dozen Congressmen, including leaders of the ruling Liberal Party, on suspicion of drug corruption--Valdivieso not surprisingly receives a constant stream of death threats. Last month the Prosecutor-General's security detail defused a powerful car bomb found parked half a block from his office. The modest lawyer takes all the danger in his stride. "These kinds of threats," he shrugs, "they all come with the territory."

Valdivieso's courageous course of action has rendered the territory more hazardous. The post of Prosecutor-General was created in 1991 in reaction to the immense corruption and fear that suffused Colombia's existing criminal-justice system. Yet Valdivieso's only predecessor, law professor Gustavo de Greiff, was so lenient with drug traffickers that he infuriated both local and U.S. officials eager to lock up Colombia's cocaine exporters for long terms. By contrast, Valdivieso, a former Education Minister and Senator, has earned their praise. Appointed by the Supreme Court to fill out De Greiff's term when he was forced to retire, Valdivieso will serve until 1996 or 1998, depending on whether the high court lets him complete a full four-year term. Until then it is almost impossible to remove him from office--a fact that must be maddening for many of Colombia's less than upstanding politicians.

A portrait hung close to his desk offers an important clue to the prosecutor's tenacity in building cases against the drug lords: Luis Carlos Galan, his first cousin, a childhood companion and an outspoken critic of drug-related corruption, was assassinated while running for President in 1989.

"Valdivieso has sent a signal against drug trafficking," says Juan Lozano, a columnist for El Tiempo, Colombia's leading daily. "He is unbribable, and he transmits that message. He's honest, fair and brave." And normally unflappable. The only time his chief lieutenant, Armando Sarmiento, recalls Valdivieso losing his temper was when a cocaine baron sent him a Christmas present.

He felt only contempt for the drug traffickers, but when his investigations moved into the political realm, Valdivieso became a threat to an old acquaintance, President Samper. He has gathered evidence suggesting that the Samper campaign accepted and concealed $6 million in election-campaign contributions from the Cali cartel. The probe intensified in July following the arrest of Santiago Medina, former campaign treasurer. The trail moved closer to the President in August when Valdivieso's men took in Botero, Samper's campaign manager.

The Prosecutor-General recently turned over to a special congressional commission boxes of documents related to allegations that Samper approved receipt of the dirty campaign money. The lawmakers will decide whether he should be impeached and tried for corruption. "We are not investigating the President," says Valdivieso. "Congress is investigating him."

Should Samper ultimately be forced from office, Valdivieso says, it would give him no pleasure: "When judicial decisions are taken, especially when they involve friends and acquaintances, one cannot feel satisfaction. The only thing that feels good is the sense that you are doing your duty."

Valdivieso is modest about his accomplishments, but they have nevertheless made him the most popular man in Colombia. Polls show his approval rating as high as 67%--the kind of number that usually results in talk of a run for the presidency. Valdivieso scoffs at the notion of such a candidacy, but in a land where politics and corruption have too often gone hand in hand, his reputation for integrity is worth millions of votes.

--Reported by Laura Lopez/Bogota