(Adds quotes from judge's ruling, Cuba reaction, details)

By Jeff Franks

HOUSTON, May 8 (Reuters) - A U.S. judge threw out all charges against anti-Castro Cuban exile and former CIA operative Luis Posada Carriles on Tuesday, allowing him to go free days before he was set to be tried for immigration fraud.

The surprise decision by U.S. District Judge Kathleen Cardone in El Paso, Texas, left uncertain the fate of Posada, who has a long history of violent opposition to Cuban leader Fidel Castro, and is viewed by many Castro opponents as a hero.

He is wanted in Cuba and Venezuela, where is accused of masterminding the 1976 bombing of a Cuban airliner that killed 73 people.

Cardone dismissed the immigration charges on grounds that the U.S. government case was based on statements it got from Posada Carriles, 79, under false pretenses.

He thought he was in an immigration interview that was actually a criminal interrogation, his lawyers said, and the judge agreed.

"The government's tactics in this case are so grossly shocking and so outrageous as to violate the universal sense of justice," Cardone wrote.

"This court will not set aside such rights nor overlook government misconduct because defendant is a political hot potato," she said in the 38-page ruling.

Her decision provoked an angry response from Cuba, which says the Bush administration has coddled Posada Carriles because of his CIA past and his support in the U.S. Cuban exile community.

"If the well-known terrorist Posada Carriles is free without charges it is the full responsibility of the White House," Dagoberto Rodriguez, Cuba's top diplomat in Washington, said in a statement.

The Bush administration, he said, "has done all it can to protect the bin Laden of this hemisphere, for fear that he can talk about the connection between the U.S. government and his terrorist activities."

DOUBLE STANDARD

Cuban leader Fidel Castro and Venezuelan President Hugo Chavez have criticized Washington for having a double standard in its war on terror, saying Posada Carriles should be charged with terrorism and murder, not immigration crimes.

Venezuela has sought his extradition for trial in the airline bombing, which occurred while Posada Carriles, a naturalized Venezuelan, lived there.

"Trying him for minor immigration infractions was a travesty of justice and was designed to fool people into believing the government was serious about prosecuting this man," said Jose Pertierra, a Washington-based lawyer representing the Venezuelan government in the extradition case.

Posada Carriles had been in U.S. custody since May 2005 after he entered the country illegally and sought asylum.

In January, he was indicted on seven immigration fraud charges accusing of lying to immigration authorities.

In theory, he faced up to 40 years in prison if convicted, but Cardone said in her ruling that most cases such as his result in a sentence of a few months.

Jury selection for his trial had been set for Friday, with opening arguments and testimony to begin on Monday.

Department of Justice spokesman Dean Boyd said the government was "reviewing the decision" by Cardone, but he did not know yet if it would appeal.

Immigrations officials took no action on Tuesday to detain Posada Carriles, but have said before they could put him in detention until his immigration status is settled.

According to news reports, a federal grand jury in Newark, New Jersey has been investigating Posada Carriles' past activities for possible indictment. The FBI took the unusual step of sending agents to Cuba to gather evidence, the Miami Herald said.

Along with the plane bombing, he is accused in Cuba of plotting a series of 1997 hotel bombings in Havana that killed an Italian tourist.

He was jailed in Panama for plotting to kill Castro during an Ibero-American summit in 2000, but was pardoned by outgoing President Mireya Moscoso in 2004.

For now, his lawyers said he would return to Miami, where he has lived with his wife since he was released from jail April 19 on bail totaling $350,000.

"He is elated," said attorney Arturo Hernandez in Miami. "He is very gratified that the system has worked."

(Additional reporting by Jim Loney in Miami, Bruce Nichols in Houston and Anthony Boadle in Havana)

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