Amid the tightest security ever imposed at the U.S. District Courthouse here, three staunchly anti-Castro Cuban exiles go on trial Tuesday in connection with the 1976 car bombing assassination of former Chilean ambassador Orlando Letelier and one of his colleagues.

The trial, which may last six weeks, could end up resembling a wedding where the minister, the bride, the bridegroom, the best man and the maid of honor never make it to the church.

Eight men, including the former head of the hated Chilean secret police known as DINA, two DINA operatives in Chile and five Cuban exiles living in the United States were indicted in the case last August after a 22-month investigation. But the three Chileans are awaiting a ruling from Chile's Supreme Court on whether they must be extradited to stand trial, and two of the Cubans have not been arrested despite a worldwide search.

Even with the five missing defendants, the trial promises to provide the fullest accounting to date of the international intrigue that culminated in the violent deaths of Letelier, a prominent and outspoken critic of the Chilean military government, and Ronni K. Moffitt, a coworker of Letelier at the Institute for Policy Studies here.

Letelier, who was jailed by the Chilean dictatorship of Gen. Augusto Pinochet that overthrew Marxist president Salvador Allende in 1973 and then was allowed to go into exile, was killed as he, Moffitt and her husband drove around Sheridan Circle on Massachusetts Avenue's Embassy Row on the morning of Sept. 21, 1976. The bomb, attached to the underside of Letelier's Chevelle, wrecked the car, dismembered him, killed Moffitt, and sent Moffitt's husband, Michael, into shock.

Government prosecutors here have alleged in an unusually detailed indictment that DINA directed the assassination of Letelier and sent operatives here to carry it out. The government's star witness has confessed to planting the bomb in the framework of Letelier's car. He is American-born DINA agent Michael Vernon Townley.

Townley, 36, a soft-spoken and intensely articulate man who had made Chile his home for the past 20 years, pleaded guilty in August in exchange for a government promise that prosecutors would recommend parole after he serves 40 months of a 3 1/2-to-10-year prison sentence.

As he is described by people who know him, Townley is an acknowledged electronic technician with the capability of building devices to detonate bombs by remote control. He had been active in commando raids against Allende. He had been charged with murder in a raid in which a night watchman was killed but the charges were dropped when Pinochet came to power and Townley became a DINA agent.

The three Cubans going on trial Tuesday -- Guillermo Novo Sampol, 39; his brother, Ignacio Novo Sampol, 40, and Alvin Ross Diaz, 46 -- allegedly had lesser roles in the killings. All three are leaders of the militantly anti-Castro exile group known as the Cuban Nationalist Movement.

Guillermo Novo and Ross are accused of conspiracy to murder a foreign official, murder by use of explosives, murder of Moffitt and destruction of Letelier's car. If convicted, they could face life imprisonment.

In addition, Guillermo and Ignacio Novo are charged with two counts of perjury in connection with testimony they gave a federal grand jury investigating the slayings and Ignacio Novo is charged with misprision of a felony -- having knowledge of a crime and failing to tell authorities about it. If convicted, Ignacio Novo faces up to 13 years in prison.

Guillermo Novo and Ross, according to the indictment, plotted with Townley to assassinate Letelier, and Novo allegedly gave Townley the explosives and a remote-controlled detonating device to carry out the bombing. Later, after the bombing, Townley allegedly briefed Ignacio Novo about the assassination.

Missing from the trial are the alleged masterminds of the plot, the three DINA agents being held at a military hospital in Chile. They are former DINA chief Juan Manuel Contreras Sepulveda, operations director Pedro Espinoza Bravo and agent Armando Fernandez Larios. In addition, Cuban exiles Jose Dionisio Suarez Esquivel and Virgilio Paz Romero are fugitives in the case.

Their absence could present problems for prosecution and defense attorneys. On the one hand, jurors in the case could have some difficulty understanding the workings of the complex scheme when only three of the alleged participants are in the courtroom although testimony at various times will involve all eight.

On the other hand, Paul Goldberger of New York, Guillermo Novo's lawyer, said that the absence of the five defendants means that they will not be available to provide information to help defend themselves and the three Cubans against the government testimony.

Goldberger said that the basic defense of the Novo brothers and Ross is that they did not participate in the alleged plot. He said they had no reason to participate in the alleged scheme and that there is no evidence they received anything of value for their alleged role

In addition, Goldberger said the defense hopes to discredit Townley's testimony by attempting to prove that he has been involved in terrorist acts throughout the world against former aides of Allende, and did not need Cubans to carry out the assassination of Letelier. But Goldberger first must convince U.S. District Court Judge Barrington D. Parker that such evidence is admissible in a case that deals only with the Letelier and Moffitt slayings.

As part of Townley's plea bargaining agreement with Assistant U.S. Attorneys Eugene M. Propper and E. Lawrence Barcella Jr. the DINA agent is required only to discuss his role in the Letelier plot.

The Novos and Ross have long been associated with anti-Castro violence. Ross was injured in the abortive 1961 Bay of Pigs invasion, while the Novos were charged in 1964 with firing a bazooka at the United Nations building as Cuban revolutionary Che Guevara was speaking there. The charges later were dropped. The bazooka shell reportedly landed in New York's East River.

In 1974, Guillermo Novo was convicted of plotting to blow up a Cuban ship anchored in Montreal, served six months in prison and was placed on probation for 30 months. But when Novo violated the terms of his probation, he was returned to jail to serve the 30 months, a term for which he currently is imprisoned.

Because of the nature of the alleged crimes, the background of the defendants and the fact that Judge Parker's and prosecutor Propper's lives have been threatned, Chief U.S. District Court Judge William B. Bryant has ordered the stringent security measures into effect.

Parker's courthouse chambers have been locked, and the judge and prosecutors in the case have been given around-the-clock protection.

Since last Wednesday, anyone entering the courthouse at Constitution Avenue and John Marshall Place NW has been required to pass through a magnetometer, show identification and open purses, bags and briefcases for inspection by Federal Protective Service officers.

U.S. marshals, who have responsibility for courtroom security, are planning to require anyone attending the trial to go through a second magnetometer. Briefcases and books will be scanned by an X-ray machine like hand-carried luggage is checked at airports.

Dogs have been sniffing through the courthouse for the past few days looking for bombs. Deputy Federal Protective Service Chief Robert W. Washington said that nothing has been found.

Armed guards will patrol the roof and grounds of the courthouse during the trial, and additional marshals will be stationed inside the courtroom, according to court officials. In the courthouse's underground garage, near the site of the recent shooting of Assistant U.S. Attorney Barry L. Leibowitz, pedestrian traffic is banned.

Forty-four news organizations, including 13 from Chile, have requested credentials to cover the trial, and reporters will have to display proper admission passes. The most stringent restrictions will be placed on members of the public.

Kris J. Sundberg, Bryant's administrative assistant, said courtroom visitors will have to wait in line for one of the 60 seats available for the public, then sign a register and hand over a piece of identification, such as a driver's license, while they listen to the trial. When they leave the trial, vistor's identification will be returned, he said.

He and others said the restrictions are a necessary precaution. "It's the fact that we've got two fugitive defendants who've got a known capability to handle explosive devices," Sundberg said. "And obviously the witnesses are not very popular with the defendants."