Rodrigo Cuadra Clachar had an appointment with some of Nicaragua's leading anti-Sandinista exiles in their suburban office here last Wednesday morning. They were waiting for him expectantly, suspiciously, already certain that he was a Sandinista agent.

Ten minutes before he was to arrive Cuadra Clachar went with another man to pick up his blue Fiat from a downtown parking lot. They paid the ticket and started to get in.

The explosion shattered windows up and down the street. The car was a twisted framework of scorched metal. Residents of the neighborhood said they thought it was an earthquake. Much of Cuadra Clachar's corpse was destroyed. His companion, now identified as Mario Gutierrez Serrano, lay critically burned nearby.

Investigators here have concluded the bomb belonged to Cuadra Clachar and was meant to be used on the people he was going to meet when it went off by accident.

For one bloody moment the secret war within the not-so-secret war against Nicaragua's Sandinistas was brought into public view.

Sandinista officials have said openly in the past few months that they had a "mole" among the people surrounding the leadership of the Democratic Revolutionary Alliance, known as ARDE, an exile force based here whose troops under the leadership of former Sandinista hero Eden Pastora are now fighting Managua's soldiers in the jungles of southeast Nicaragua.

In an interview six weeks ago, Jaime Wheelock, one of the top nine commanders of the ruling Sandinista National Liberation Front, pulled pages of handwritten documents from his desk drawer in Managua and waved them before reporters as an example of this agent's reporting.

There is now widespread speculation that Cuadra Clachar was that agent.

But, on the other hand, in the convoluted world of counterespionage such openness about such a sensitive source would seem to suggest that the source does not exist. The Cuban and East German advisers who have helped put together Nicaragua's State Security apparatus are known for the extreme security precautions they preach.

Advertising the presence of an imaginary agent, however, could sow suspicion and fear in an exile movement already riddled with doubts.

Most of the top leaders in ARDE are former members of the revolutionary government in Managua and there is always the possibility that some are still working for it. New recruits join all the time while others continue feeling out the possibilities.

Cuadra Clachar, 31, had been in contact with ARDE at least since the beginning of the year, according to members of the organization.

Born in northern Costa Rica of Nicaraguan parents, he was reportedly a deputy minister of commmunications in the first year of the Sandinista government. A chemist, he went on to work with the Health Ministry and state-owned laboratories.

Some ARDE leaders, including Pastora in an interview with local reporters after the bombing last week, said they had suspected Cuadra Clachar for months.

Apparently those suspicions were not confirmed until mid-June when ARDE political director Alfonso Robelo went to Washington. Part of his aim there was to win financial support for ARDE's rebels from the United States whose Central Intelligence Agency reportedly has funded much of the "secret war" being waged by a separate rebel faction working out of Honduras in the north.

But another of Robelo's goals, he said, was to check with intelligence sources about what they knew of efforts to infiltrate his organization.

At least one defection by a senior member of Nicaraguan State Security in recent months reportedly has provided U.S. sources with detailed information about the workings of that organization and some specifics about its current informants.

In an interview here after the bombing Robelo said that a source in Washington gave a cryptic but seemingly positive identification of a double agent in the ARDE ranks.

Robelo said he was told, "There is a government man from Nicaragua who is trying to infiltrate your organization. He has been to Costa Rica five times in the last six months. He has been in contact with Eden, even been to his house." The voice of the agent, Robelo was told, had been heard on tape-recorded messages.

"I checked with our own intelligence people," said Robelo. "There was no doubt that this was Rodrigo Cuadra."

Robelo speculated that Cuadra Clachar intended to kill several of ARDE's leaders with the bomb, if possible. If not, he might try to blow up the ARDE office.

The Nicaraguan government steadfastly denies charges that it is involved in espionage, had any connection with the bombing or, at the time of his death, with Cuadra Clachar. A Foreign Ministry communique denounced what it termed "veiled and-or direct accusations" intended "to discredit us" and pointed out that Cuadra Clachar was apparently a member of ARDE and not the Sandinista government when the bomb exploded.

The presence of the exile leadership here is a political problem for Costa Rican President Luis Alberto Monge. An explosion at ARDE "could have caused all kinds of problems," Robelo said.

Costa Rica is officially neutral toward Nicaragua's internal conflicts. Its people, who value their peaceful society and successful democracy, are increasingly fearful that the country will be drawn into the violence of its Central American neighbors.

But anti-Sandinista exiles operate openly in the capital. With their progressive but noncommunist ideas, much like those of many Costa Ricans, they have a great deal of sympathy from the public and in some parts of the government.

While the Sandinistas' response to the "counterrevolutionaries" working out of Honduras has been mainly military on battle lines near the northern border, the answer to the anti-Sandinistas operating here appears for the most part to have been covert and concentrated right in the capital.

Costa Rica's response, sometimes with U.S. prodding, has been diplomatic retaliation. Some members of its police force have been given U.S. training to deal with terrorism.

Early last year Costa Rica broke relations with Cuba amid suspicions that Havana was running a major espionage network here and after what former U.S. Ambassador Francis J. McNeil called his "constant, constant pushing."

Each new demonstration that this web of covert action and reaction is in operation here sends tremors, many with xenophobic overtones, throughout this normally open and friendly country.

A crackdown on foreigners, particularly Nicaraguans, reportedly has been under way since the bombing last week.

The respected daily newspaper La Nacion, citing "confidential" sources, said that Sandinista State Security assigns agents to ride buses coming from Managua to San Jose to report on the activities of travelers.