JUAN O. TAMAYO Herald Staff Writer

One of the bloodiest and most confounding mysteries left from an era when CIA, Sandinista and Cuban spies fought a nasty little war in Nicaragua appears to have been solved.

For nine years, CIA agents have borne the brunt of the blame for the suitcase bomb at a jungle news conference that killed eight people and shattered the left leg of contra leader Eden Pastora, the famed Commander Zero.

Costa Rican prosecutors have indicted two CIA operatives -- a Cuban exile living in Miami and a 71-year-old farmer from Illinois -- as the masterminds of the attack and are demanding they be extradited from the United States.

But new evidence unearthed by The Herald, Argentine and U.S. journalists, during a lengthy investigation that included interviews with former CIA and KGB agents points strongly in another direction -- toward Nicaragua's leftist Sandinista Front.

The bomber: Vital Roberto Gaguine, a leftist Argentine guerrilla killed in 1989 in a virtual suicidal assault on an Argentine army barracks.

A thumbprint found in one of the bomber's official documents matches perfectly the thumbprint taken of Gaguine in Argentina two decades ago.

And in Miami Beach, the father and brother of Gaguine identified a crucial photograph of him.

Gaguine, a member of an Argentine guerrilla group that based itself in Nicaragua, worked for Sandinista intelligence in a number of foreign operations like the 1980 assassination of dictator Anastasio Somoza, two former group members told The Herald.

Sandinistas deny any role in the blast. "If you asked me if I knew anything about this attempt, I would say no," Tomas Borge, Sandinista minister of the interior and intelligence chief at the time of the bomb, said last week.

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The date was May 30, 1984, and the place was a one-hut guerrilla base called La Penca, on the Nicaraguan side of the cafe-au-lait San Juan River that marks Nicaragua's southern border with Costa Rica.

Eden Pastora, the most charismatic of the contra leaders financed by the CIA to fight against Sandinismo, had just begun a news conference and asked the journalists to stand around a chest-high table where peasants usually stood to eat.

As he spoke, a bomb left under the table exploded, spewing a deadly fan of peanut-sized steel balls that scythed off half a dozen legs.

Pastora suffered a shattered leg and flash burns over most of his body. Five of his rebels and three journalists died: Linda Frazier, 38, a reporter for Costa Rica's English-language Tico Times; and Costa Rican TV crewmen Jorge Quiroz and Evelio Sequeira.

From the beginning, police had a superb suspect. But they lacked his real name.

The bomb had gone off in the camera case of a handsome but mysterious man in his 30s. He had identified himself as a news photographer and traveled on a stolen Danish passport in the name of Per Anker Hansen. He was photographed moving away from the table.

After the remote-triggered blast, "Hansen" was among a dozen wounded journalists evacuated to a hospital in Costa Rica. He was treated for minor scratches. While there, reporters interviewed and photographed him again.

But only hours later, he simply vanished.

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There was never a shortage of suspects when it came to people who might wish to see the temperamental Pastora dead.

A hero of the Sandinista revolution that toppled Somoza, Pastora later broke with the Sandinistas. He denounced them as Marxists and accepted CIA cash to arm a contra faction based in southern Nicaragua.

But he also quarreled with CIA agents who ordered him to merge his force with a more conservative CIA-backed group. He bickered with his Indian allies, feuded with political backers and fought with his mistresses.

"Pastora was a pain in the butt to everyone," said a former U.S. ambassador to Costa Rica, Curtin Windsor. "The Sandinistas, the CIA, his wife. You name it, and they might have tried to kill him."

True.

But the bombing occurred in the midst of the war between the CIA-backed contras and the Sandinistas. Given the CIA's history of nasty meddling in Latin America, it was easy to believe that the CIA or its unsavory agents had tried to kill Pastora for disobeying its orders.

Spearheading the CIA-did-it campaign were Tony Avirgan, a U.S. journalist wounded in the blast, and his wife, Martha Honey, both political liberals opposed to Reagan policies.

They wrote a 166-page report charging "Hansen" was a Libyan-born, right-wing hit man paid $50,000 of the CIA's money to kill Pastora. And they filed a $23 million suit in federal court in Miami accusing 29 contras and CIA operatives.

Judge James Lawrence King not only threw out the lawsuit in 1988 as unsupported, he ordered them to pay $1.3 million in defense attorney fees.

Even so, a young Costa Rican prosecutor indicted two CIA operatives a year later. They are now lying low in South Florida and the Midwest, hoping to avoid extradition.

The Herald's investigation, however, turned up hints of Sandinista involvement in the bombing early on.

In an interview in Moscow after the collapse of the Soviet Union, a retired KGB lieutenant colonel said Soviet agents in Managua and Havana had reported soon after the blast that it was the work of the Sandinistas.

The KGB agent, who worked in Latin America from 1981 to 1986, told The Herald that the bombing was carried out with the help of "a Cuban intelligence agent named something like 'Carlos Montoya.' "

Although no "Carlos Montoya" could be found in Nicaragua, the KGB agent may have meant Renan Montero, the head of Sandinista intelligence in the early 1980s. Montero was in fact a colonel in the Cuban Interior Ministry's elite Special Forces. Sandinistas issued him a Nicaraguan passport.

Because the Sandinistas lacked any sort of sophisticated spy network in the early 1980s, Montero "contracted" some of his foreign jobs to a faction of Argentina's leftist Peoples' Revolutionary Army.

This is how Vital Roberto Gaguine , the bomber at La Penca, got involved.

The Argentine faction has acknowledged murdering a contra leader in Honduras in late 1979 and assassinating Somoza in Paraguay in 1980 as a "revolutionary favor" for the Sandinistas. Its leader, Enrique Gorriaran Merlo, participated in both hits and pumped some 30 bullets into Somoza.

Borge, the onetime Sandinista interior minister, admitted last week that Montero had used foreign agents: "There were Guatemalans, North Americans, Venezuelans . . . Argentines," he said. "It was a strong unit, helped a little by the solidarity of the people who came to us."

The Herald's first tip on the true identity of "Hansen" came this April from a former member of the Argentine faction who defected and now lives in Europe.

He said he had long ago recognized newspaper photos of "Hansen" as a faction member. He knew him only by the pseudonyms of "Martin" -- and "Martin the British" because of his fluent English.

"Martin," he said, lived in Managua through the early 1980s and was trained in weapons at a Sandinista militia base in Managua. Oddly enough, the commander of the Sandinista militias at that time was Pastora.

While The Herald's source said he had no specific knowledge of the La Penca bombing itself, he said he had heard from faction members about an Argentine plot to assassinate Pastora in Costa Rica the same year as La Penca.

His description of "Martin," including details about his wife who committed suicide, led to a tentative identification with the assistance of Argentine security officials and several Buenos Aires journalists interested in the case.

"Martin," they concluded, might be Vital Roberto Gaguine, a former medical student born June 23, 1953. He had been a high school exchange student in the United States and later studied in London in the early 70s.

"Martin" returned from Nicaragua to Argentina in about 1987. He was killed Jan. 23, 1989, while leading one of the four squads that assaulted the La Tablada barracks just outside Buenos Aires, according to the book Gorriaran, La Tablada and the 'Intelligence Wars' in Latin America by Argentine journalists Juan Salinas and Julio Villalonga.

Last week in Buenos Aires, federal police gave reporters a copy of Gaguine 's fingerprints -- taken when he registered for his national identification card in 1972 at the age of 19.

The Herald gave those prints to a fingerprint expert hired in Miami. And freelance journalist Douglas Vaughn gave the expert a right thumbprint he had discovered in a document filed by "Hansen" with Panamanian authorities in 1982.

The result: "A 100 percent match, no questions," the expert said.

Vaughn then showed Gaguine 's father and brother, Samir and Eduardo, news photographs taken of "Hansen" just before the bombing.

"No, no doubt at all it was him," the father said later.

The identification of the bomber appears to open the final chapter of the La Penca mystery.

Two years ago, Pastora said he suspected the CIA was behind the bombing. But on Thursday, he said in a telephone conversation he had "always felt" the bomber was an Argentine he had seen in Managua.

Felipe Vidal, the Miami Cuban indicted in Costa Rica and hiding in South Florida, said he now expects the charges and extradition request will be dropped.

"I have always said that the charge against me and the CIA was disinformation, and this proves it," he said.

And John Hull, the aged Illinois native who grew oranges for 18 years in Costa Rica, plans to reclaim his farmlands as soon as the Costa Ricans drop their charge.

Not so fast, say those who still hold out the possibility of a CIA-did-it conspiracy.

Avirgan, the journalist who filed the lawsuit in Miami federal court, said the identification of "Hansen" as Gaguine raises as many questions as it answers.

What about all the evidence he gathered over the past nine years on the right-wing CIA operatives who hired the Libyan hit man, he wants to know.

Even the CIA was troubled enough by some of the allegations that it conducted an in-house investigation in 1986, including giving lie detector tests to some of the operatives named in Avirgan's lawsuit.

Vaughn, an investigative journalist hired by Avirgan's lawyers, said he had little doubt that "Hansen" was Gaguine, but believes it's possible there were two parallel attempts against Pastora, one by Gaguine and one by right-wing CIA operatives.

Perhaps, Vaughn argues, that it is even possible that right-wing factions had infiltrated the Argentine group and manipulated Gaguine from behind the scenes.

Maybe. But after nine years, one detail of the La Penca bombing appears certain:

It was carried out by an Argentine leftist who lived in Managua, was trained by the Sandinista militias and died in an attack against an Argentine army barracks.

Caption: color photo: Eden Pastora (a); photo: John HULL, Contra leader Eden Pastora flashes a victory sign, Fake identity Vital Roberto Gaguine alias Hansen photographed in explosion's aftermath, From Gaguine : 1972 Thumbprint taken when he registered for his national identification card in Argentina, 1982 From Hansen: Thumbprint filed with Panamanian authorities under fake passport (4-n)