By Juan Forero

BOGOTA, Colombia, March 19 -- Sitting in a dreary 7-by-5-foot cell, Rafael Garcia predicts that he'll soon be murdered. It's a common threat in one of Colombia's toughest prisons, but it's made all the more real for the uncommon prisoner.

Garcia is a star witness for prosecutors, revealing secret links between Colombian officials and right-wing paramilitary groups. His testimony has helped trigger the biggest political scandal faced yet by the government of President Alvaro Uribe, the Bush administration's closest ally in Latin America and recipient of more than $4 billion in American aid.

Once a high-level official in the government's intelligence agency, Garcia has outlined for investigators how the intelligence chief, Jorge Noguera, funneled classified documents to paramilitary commanders, and how those commanders rigged elections to place their allies in Congress, giving an organization designated a terrorist group by the State Department unprecedented influence in government.

Along with secret paramilitary documents and other witness testimony, the information provided by Garcia has led to the arrests of Noguera, eight congressmen and the governor of Magdalena state, Trino Luna. Another 15 current and former congressmen, as well as other local officials and military officers, are under investigation.

Garcia is serving an 11-year prison sentence for money-laundering, conspiracy, and falsifying and destroying documents.

Despite the information he has furnished investigators, Garcia said in an interview Saturday that authorities have denied his frequent petitions to be placed in a witness protection program. Already, a handful of mid-level paramilitary commanders with deep knowledge about the paramilitary structure have been murdered.

"As long as I'm in a Colombian jail, my death is only a matter of time," Garcia, 43, said as he sat on the narrow bed in his cell at La Picota prison in Bogota, a bare light bulb hanging over his head.

Garcia recently wrote Attorney General Mario Iguaran, saying that he would withhold further testimony unless the state offers him, his wife and their son more protection. He said he is not trying to blackmail the government. "This is to preserve my life and the integrity of my 11-year-old son, who is the most important thing I have in this life," he wrote in the letter.

Officials at the attorney general's office and the Interior Ministry did not return several phone calls on Monday.

U.S. authorities are also interested in hearing from Garcia; his cooperation is considered important to understanding drug trafficking through Venezuela and alleged support from U.S. corporations for paramilitary groups.

Garcia has information, for instance, about ties between paramilitary groups and Chiquita Brands International. Last week, the Cincinnati-based banana company agreed to pay $25 million to the Justice Department after admitting to having paid paramilitary groups $1.7 million for protection in the northern Uraba region.

In Alabama, a court is expected to rule soon on whether Garcia's testimony can be admitted in a lawsuit that accuses Drummond Coal Co. of having paid paramilitary members to kill three union leaders. The company denies the accusations.

"This guy is probably the most endangered guy in all of Colombia, so we're worried about it," said Terry Collingsworth, an attorney representing Drummond workers.

Garcia was head of information services at the Administrative Security Department, or DAS, as the intelligence agency is known by its Spanish initials. He had access to files on drug traffickers, information about investigations of paramilitary groups and other important documents. In 2005, investigators discovered that Noguera and other DAS officials were providing the United Self-Defense Forces of Colombia, the loose coalition of paramilitary groups known as the AUC, with secret information while illegally destroying case files on the organization's drug trafficking and other crimes.

The AUC, declared a terrorist organization by the United States, had for years massacred peasants and leftist activists while funding its war against Marxist rebels with cocaine trafficking. Garcia, a university-educated carpenter's son, became a member of the Northern Bloc of the AUC, a powerful militia run by Rodrigo Tovar.

In 2002, Garcia was among those who helped orchestrate what he calls a "massive electoral fraud" that enabled paramilitary groups to put handpicked candidates into Congress. Those congressmen, close allies of the Uribe administration, helped approve a law that the United Nations criticized as overly generous to paramilitary commanders who took part in a three-year disarmament of the group. Last May, the country's Constitutional Court struck down several articles of the law, ending some concessions shielding the warlords.

Uribe has steadfastly defended his administration, saying he has moved aggressively to dismantle the paramilitary groups. He has also vigorously attacked opposition politicians who have raised questions about his government's ties to paramilitary groups, calling some of them "terrorists in business suits" who ignored the inroads Marxist guerrillas made into Colombian society.

Garcia's credibility has remained largely unscathed. He believes it is because of his privileged position in the DAS and his AUC membership. "I was part of his government," he said, referring to the Uribe administration. "I was, am and will always be of the right. They can't discredit me like they do all the others."

Garcia said he still believes in the paramilitary groups' professed mission: to wipe Colombia free of Marxist guerrillas. But he also now believes that the paramilitary groups, by sacking public hospitals, extorting money and assassinating opponents, have done the country harm.

"I could have said just one thing and gotten benefits," Garcia said, referring to his cooperation with authorities. "Why did I say so much? Because the region we hurt so much needs another chance."

He said he also wants to leave a different legacy. He recalls that his father, Adaulfo Garcia, who died of a heart attack last year, had been a humble, honest man.

"My father worked hard to instill working-class values and for some reason, I didn't carry those same values," he said. "So I do this in honor of his memory and to give my son another legacy."