Guillermo Novo Sampol, 39, one of two men found guilty yesterday of killing Orlando Letelier, had left Cuba long before the revolution. He and his brother, Ignacio, came to the United States in 1954, but after Castro triumphed at the end of the decade the Novos became as dedicated to terrorism against his regime as any of the thousands of exiles who fled their homeland.

Ten years after they first came here they were charged with firing a bazooka at the United Nations building while Cuban revolutionary leader Che Guevara was speaking there, but the charges later were dropped.

In 1974, after 20 years outside Cuba, Guillermo still was fighting for what he considered the liberation of his country. He was convicted of plotting to blow up a Cuban ship anchored in Montreal. He served six months in prison and was placed on probation for 30 months.

After violating the terms of his probation he was returned to jail, but during his period of freedom, the jury concluded yesterday, he helped plan and execute the bombing that claimed the lives of Letelier and his associate, Ronni K. Moffitt.

Alvin Ross Diaz, convicted yesterday of murdering former Chilean ambassador Orlando Letelier, is a veteran of political violence.

Long after he was injured in the abortive Bay of Pigs invasion in 1961, he continued to immerse himself in the deadly fanatical fringe of this country's Cuban exile community, a circle of men who still believe they can win back their country through terror.

Over the years the focus of their violence spread to include members of other Marxist regimes throughout the world.

One witness at his trial testified that Ross, 46, dreamed of sending explosivepacked motorboats to sink Russian ships in American harbors by remote control. The same witness, who had known Ross in a New York jail, said he had boasted of once firing a bazooka at Fidel Castro in a motorcade, only to kill "some people" in a car behind the Cuban leader.

When a bomb was needed to assassinate Letelier, a ranking official in the fallen Chilean communist regime, Ross bragged that he was one of the men who helped build it by contributing two wires, according to prosecution witnesses in the case.

Ignacio Novo Sampol, 40, obeyed the code of silence vital to any terrorist and was convicted yesterday of perjury and failure to report a crime.

He had told a grand jury that he knew nothing about the murders of Orlando Letelier and Ronni K. Moffitt -- crimes for which his younger brother Guillermo and his friend Alvin Ross Diaz were convicted yesterday. The jury concluded that Ignacio lied.

In 1964, when Ignacio Novo and his brogher allegedly fired a bazooka at the United Nations headquarters in New York City, the projectile reportedly fell into the East River.

In 1976 he became one of the official new "military leaders" of a small group of Cuban exiles committed to violence, subsequently endorsing such acts as the bombing of a Cuban commercial airliner in which 73 people died.

Though never involved with the Bay of Pigs and never trained by the CIA, as so many members of the radical Cuban movement were, Ignacio and his brother reportedly were considered by other Cuban exiles to be two of the wilder and more disreputable men in their ranks. CAPTION: Illustration 1, 2, Picture 1,

Below are profiles of the three anti-Castro Cuban exiles who were convicted yesterday of charges stemming from the 1976 bombing assassination here of former Chilean Ambassador Orlando Letelier.

Two of the men -- Guillermo Novo Sampol and Alvin Ross Diaz -- were found guilty of killing Letelier, while the third -- Ignacio Novo Sampol -- was convicted of perjury and failure to report a crime.

Guillermo Novo and Ross face mandatory 20-year prison terms and could receive life sentences.

In its verdict, the jury accepted the contention of prosecutors that the Chilean secret police, once known as DINA, had masterminded the murder plot and ordered Letelier's slaying.; Sketches by Joan Andrew for The Washington Post