By ANN LOUISE BARDACH and LARRY ROHTER New York Times Service

GUATEMALA CITY -- During the summer of 1997, bomb explosions ripped through some of Havana's most fashionable hotels, restaurants and discotheques, killing an Italian tourist and sowing confusion and nervousness throughout Cuba.

At his office in the mountains of Central America, a Cuban-American businessman named Antonio Jorge ``Tony'' Alvarez was certain he knew the answer. For nearly a year, he had watched with growing concern as two of his partners -- working with a mysterious gray-haired man who had a Cuban accent and multiple passports -- acquired explosives and detonators, congratulating each other on a job well done every time a bomb went off in Cuba.

What is more, Alvarez overheard the men talk of assassinating Cuban President Fidel Castro at a conference of Latin American heads of state to be held on Venezuela's Margarita Island. Alarmed, he went to Guatemalan security officials. When they did not respond, he wrote a letter that eventually found its way into the hands of Venezuelan intelligence agents and FBI officials in the United States.

Venezuelan authorities reacted energetically to the information, searching for explosives on the island where the meeting was to be held. But in the United States the letter elicited what Alvarez described as a surprisingly indifferent response.

No follow-up call

An agent in the Miami FBI office reached him by phone, Alvarez recalled in recent interviews, and said a colleague would call soon to arrange to speak with him. In the meantime, he urged Alvarez to leave Guatemala immediately.

``He told me my life was in danger, that these were dangerous people, and urged me to get out of Guatemala,'' said Alvarez, a 62-year-old engineer. ``But I never heard from him again.''

Had the FBI met with Alvarez, agents would have heard a remarkable tale about the anti-Castro underworld.

They would have learned that the gray-haired man was Luis Posada Carriles, an anti-Castro exile who has devoted his life to overthrowing the Cuban government.

They also would have heard about possible links between the plotters in Guatemala and Cuban exiles living in Union City, N.J., who Alvarez said were wiring money to the plotters. That allegation raises questions about whether American laws were broken in the Cuban hotel bombings.

Agent `a very good friend'

John Lewis Jr., an FBI assistant director in charge of national security issues, declined to comment on Alvarez's letter or whether any agent had spoken with Alvarez. FBI officials would say only that as a matter of policy they respond to reports of possible acts of violence anywhere. But Alvarez says the FBI showed a lack of curiosity about the bombings. And Posada, who acknowledged in an interview that he had directed the operation, said he had no indication that the FBI was investigating him.

In the interview, Posada described the FBI agent who had phoned Alvarez in Guatemala, Jorge Kiszinski, as ``a very good friend'' whom he had known a long time. ``He's going to retire this year,'' Posada said. ``He has helped not just me but many people.''

Lewis of the FBI said such a friendship between the two men was implausible. ``Agent Kiszinski has had two contacts with him in his entire life, the last of which was a number of years ago,'' he said.

Bitter experience

Posada expressed confidence that the FBI was not examining his operations in Guatemala, because ``the first person they would want to talk to is me, and nobody called.'' In addition, he said, no one from the bureau has tried to interview his collaborators. ``I would know,'' he said.

Alvarez, in contrast, has been embittered by his experiences as a whistle-blower and believes that Posada has long provided information to American authorities.

``I think they are all in cahoots, Posada and the FBI,'' he said. ``I risked my life and my business, and they did nothing.''

In his letter alerting Guatemalan authorities to the plot, Alvarez wrote that while he opposed the Castro government and communism, ``I believe that terrorism is not the way to resolve the Cuban (or any other) situation.''