BY GLENN GARVIN

 PANAMA -- Cuban officials claim that four Cuban Americans arrested here last
 week planned to shoot down Fidel Castro's plane with two surface-to-air missiles
 and are demanding to know why Panamanian police haven't been able to find the
 weapons.

 ``The Cubans are pressuring us, pressuring us hard, saying, `Where are these
 things?' '' a Panamanian security official told The Herald. ``But so far we haven't
 come up with them.''

 Panamanian investigators have discovered 18 pounds of plastic explosives they
 believe are linked to the four accused conspirators, but have yet uncovered no
 evidence that they had surface-to-air missiles, the official said.

 ``All we've got so far is the Cubans' word for it,'' he said. ``But obviously we've
 been working with their information from the beginning, and so far it's been pretty
 good.''

 A former Panamanian diplomat with close connections to Castro's government
 confirmed that Cuban intelligence reported that there was a plan to fire rockets at
 Castro's plane.

 The allegation about the missiles is the latest surprising twist in a story that has
 had plenty of them since the four men were picked up Friday shortly after Castro
 announced the assassination plan during the Ibero-American summit here.

 In other developments:

   The Cuban government revealed that it has asked for the extradition from
 Panama of all four men, not just veteran anti-Castro conspirator Luis Posada
 Carriles as had been previously thought.

 But officials here, citing their difficulties getting Castro to surrender fugitives from
 Panamanian justice who have sought refuge in Havana, privately said no one will
 be extradited to Cuba.

   Panamanian police revealed that two more suspects -- a Cuban-born
 businessman who is a long-time Panama City resident, and a Panamanian
 chauffeur -- were arrested Monday in connection with the alleged plot.

   Cuban officials released a detailed dossier on the four accused conspirators,
 including such arcane points as the numbers of the cellular telephones they use
 in Miami.

 The accuracy of the information has Posada Carriles' associates in Miami
 ``running scared,'' said one of them, because they feel certain their group has
 been penetrated by Cuban intelligence.

   The four men still haven't been formally charged with anything.

   Security officials confirmed that one of the Cuban-Americans arrested last
 week, who entered Panama with a U.S. passport in the name of ``Manuel Díaz,'' had a
 second one identifying him as Gaspar Jiménez, a Miami man with a long record
 of anti-Castro violence.

 Cuban Foreign Minister Felipe Pérez Roque also identified ``Manuel Díaz'' as
 Jiménez during a speech on Havana television, and visitors to the cellblock where
 the Cuban Americans are being held said the man was answering to the name
 Jiménez.

 Pérez Roque also said a diplomatic note delivered to the Panamanian foreign
 ministry Saturday night requested the extradition of not only Posada Carriles --
 who faces a death sentence handed down in absentia by Cuban courts more than
 two decades ago -- but Jiménez as well as Pedro Remón and Guillermo Novo, the
 other two Miami men under arrest.

 "I think that once they know the whole story, the Panamanian people, their
 authorities and their quite diligent and professional chiefs of security will
 understand well the expectations of our people and the interest with which they're
 following this situation,'' Pérez Roque said. "The victims' families demand justice,
 and the international community understands that all the necessary evidence
 exists to try these assassins of innocent civilians.''

 But Panamanian President Mireya Moscoso and her top advisors have been
 extremely critical of the information in Cuba's extradition request, and one official
 said privately Tuesday that there's already widespread agreement at the top of the
 government that extradition is out of the question.

 Cuba, the official noted, has for years rejected all requests to send accused
 Panamanian criminals home, including two former captains in Gen. Manuel
 Noriega's army accused of murdering fellow officers who plotted a coup against
 him.

 "We've asked and asked, and they just ignore us,'' said the official. "It has to be
 a two-way street.''

 Several officials, as well as criminal defense attorneys with broad experience in
 extradition cases, also noted that Panama has a long record of refusing to
 extradite anyone who might face the death penalty in another country, and in
 recent years has refused extradition requests even to countries with life
 imprisonment.

 The two men arrested Monday were identified as César Matamoros, a Cuban-born
 businessman, and José Hurtado, a Panamanian who police said had been driving
 Posada Carriles around the capital.