A university professor acknowledged Friday that he was a "collaborator" with Cuba's intelligence service but insisted he had distanced himself from the communist government by the time he confessed details of his work to the FBI in the summer of 2005.

Florida International University psychology professor Carlos Alvarez, who is charged with failing to register with the U.S. as a Cuban agent, acknowledged communicating with Cuban officials using a short-wave radio, using sophisticated encryption techniques and a code name, which was "David."

"I was collaborating, basically, sharing insights and information with the Cuban government for some years," Alvarez testified in a federal court hearing. "Information that I felt was pertinent."

But Alvarez said he quit providing information to Cuba by 1998, well before he was approached by two FBI agents in June 2005 at a Miami supermarket about his involvement. Alvarez also insisted he was never a covert Cuban agent.

"I was not an agent of the Cuban government. I was a collaborator, which is very different," he said.

Alvarez's surprise admission came on the third day of a hearing on a motion filed by his lawyer contending that the FBI promised him immunity from prosecution if he fully confessed his involvement with Cuba. The motion also contends that the confession was coerced, even though the FBI agents repeatedly told Alvarez he was not under arrest and was free to leave the interviews on June 22-23 and July 1, 2005.

If Alvarez prevails on the motion, his confession to the FBI will not be admissible at trial. Prosecutors, however, also have other evidence in the case, including material from telephone wiretaps and a listening device in the Alvarez's home.

Prosecutors cannot use Alvarez's admission in Friday's hearing against him in trial, according to previous U.S. Supreme Court rulings. But Alvarez's lawyer, Steve Chaykin, objected to any further testimony about details of the FBI confession.

"We've acknowledged that the statements that he made did incriminate him," Chaykin said.

Alvarez, 61, and his 56-year-old wife, Elsa, have both pleaded not guilty to charges of being unregistered Cuban intelligence operatives for more than two decades, reporting mainly on activities of Cuban-American exile groups in Miami and on U.S. political developments.

Prosecutors say Carlos Alvarez continued his work until at least 2004, not 1998 as he claimed Friday. Brian Frazier, the assistant U.S. attorney prosecuting the case, cited recent meetings between two Cubans, including one Frazier described as an intelligence officer.

In videotape transcripts of his FBI interviews, Alvarez repeatedly agreed with the FBI agents when they said his confession was voluntary and that he was free to go anytime. Alvarez testified Friday that he feared that contradicting the agents would subject him to criminal prosecution and result in "a scandal" for his family.

"It wouldn't make any sense to volunteer information to law enforcement if I felt it was going to be used against me," Alvarez said. "I was very fearful."

Alvarez also repeated that he believed he was given a promise of immunity from prosecution in return for his confession. The two FBI agents, Albert Alonso and Rosa Schureck, testified earlier that no such commitment was made.

The agents also testified that their ultimate goal was to turn Alvarez into a "double agent" who would spy for the U.S. against Cuba. Alvarez, however, said Friday that he had not heard that before this week's hearings.

"I was surprised when I heard about it in court," Alvarez said. He did, however, bring up the issue himself and rule out doing such spy work with the FBI agents, according to the transcripts.

U.S. Magistrate Judge Barry Garber did not indicate when he would issue a ruling, which also would be subject to final review by another federal judge. The Alvarezes are scheduled to stand trial in early 2007.