BY GLENN GARVIN

 PANAMA -- One of the four Cuban exiles arrested in Panama last year in
 connection with an alleged plot to kill Fidel Castro told investigators in an
 ``informal conversation'' that he planned to assassinate the Cuban leader with a
 car bomb but changed his mind at the last minute, Panamanian authorities say.

 In a series of interviews with The Herald this week, the officials also offered the
 most complete account yet of the events leading up to the arrest of four alleged
 conspirators last November -- including a disclosure that Castro himself
 personally delivered a key piece of information to Panamanian investigators.

 According to the officials, 70-year-old Luis Posada Carriles, a veteran of countless
 previous anti-Castro conspiracies, told investigators that he called off the plan to
 kill Castro during a Latin American summit in Panama because too many
 innocent people would have been harmed as well.

 Posada Carriles and three Miami men are in jail here on charges of illegal
 possession of explosives and conspiracy.

 However, Panamanian officials -- who spoke on condition that they not be
 identified -- admitted their case against the men is extremely weak and predicted
 that they will be acquitted at trial.

 NO EXTRADITION

 Extradition of the men to Cuba, which Castro has demanded, has been flatly ruled
 out at the highest levels of the Panamanian government, the officials said. In
 addition to Posada Carriles, the others are Gaspar Jiménez, 65; Pedro Remón,
 56; and Guillermo Novo, 61.

 The four were arrested Nov. 17 during the Ibero-American Summit, after Castro
 warned Panamanian security forces that Posada Carriles was in Panama City to
 assassinate him.

 According to the version of events provided by Panamanian officials, the Cubans
 brought as many as 100 security agents to Panama to prepare for the summit;
 some of them were under cover as teachers and businessmen.

 But the Cuban government never offered any evidence that Posada Carriles
 was in Panama until Castro himself was passing through the reception line at
 Panama City's Tocumen International Airport Nov. 17.

 As he shook hands with Panamanian officials, Castro said he needed to meet
 with them at his hotel -- ``I've brought some information from Havana for you.''

 CASTRO'S NEWS

 About half an hour later, at the hotel, Castro told the Panamanians that Posada
 Carriles had entered Panama earlier in the month and was staying at a Panama
 City hotel under an assumed name. The Panamanian officials immediately
 dispatched a team of policemen to the hotel.

 But what they did not know was that, immediately after meeting with them,
 Castro gave a press conference at which he revealed Posada Carriles' presence
 to the entire world.

 ``If any of those four guys had been watching television, they would have gotten
 away,'' said one Panamanian official.

 Instead, two of the men -- Novo and Remón -- were arrested in the street outside
 the hotel when they tried to run after spotting the police. Posada Carriles and
 Jiménez were arrested upstairs in their hotel rooms, where they had just awoken
 from naps.

 GROUNDS FOR BELIEF

 ``That's why I'm inclined to believe Posada Carriles when he says the plot had
 been called off,'' said a Panamanian official.

 ``He wasn't acting like a guy who was stalking Castro. Instead of watching TV,
 trying to figure out Castro's plans, he was sleeping. That doesn't sound like an
 active assassination plot to me.''

 The three Miami men continue to insist that they came to Panama merely to
 protest Castro's visit, authorities say. But Posada Carriles, in informal
 conversations, has admitted that he was here to kill Castro, according to the
 same sources.

 They said he told them the plan was to pack an automobile with plastic
 explosives and then detonate it as Castro's motorcade passed. But he decided
 that too many other people would be killed, the authorities said, and decided to
 drop the idea.

 Because Posada Carriles talked about the plan but would not provide a sworn
 statement to investigators, however, his declarations cannot be used as evidence.

 And, the Panamanian officials admitted, the links between the accused plotters
 and the only significant physical evidence in the case -- a briefcase full of plastic
 explosives found in a rental car they were using -- are too weak to win a
 conviction.

 ``I think that's going to be a pretty difficult case to sell in a courtroom,'' said one
 official. ``I don't see any way we'll get a conviction.''