One of two men serving time in federal prisons in connection with the 1976 assassination of former Chilean ambassador Orlando Letelier was inadvertently released from jail Monday. He is currently being sought by federal agents.

Prison officials confirmed yesterday that Guillermo Novo Sampol, who was serving 54 months for lying to a grand jury investigating the car bombing assassination, was released after the officials confused him with his brother, Ignacio Novo Sampol, another Letelier defendant. Ignacio had been ordered freed by a U.S. Distict Court judge last week.

Late yesterday, federal officials said they were focusing their search for Guillermo Novo in Union City, N.J., where his family lives.

"We're trying to reach him," said his lawyer Paul Goldberger in New York. Goldberger added that he had learned of the unexpected release of his client when the Novo family telephoned him yesterday to give him the news.

"If it's a mistake, he'll go back," Goldberger said.

Ignacio Novo had pleaded guilty to lying to the same grand jury to which his missing brother had lied. Last Tuesday Judge Barrington D. Parker ordered that Ignacio's sentence should be equal to the time he had already spent in jail.

According to prosecutor E. Lawrence Barcella Jr., when the U.S. marshals in Washington received Parker's order in Ignacio's case Friday, they called the federal prison at Danbury, Conn., where Guillermo Novo was serving his time.

Barcella said that they asked, "Do you have a Novo Sampol?" When Danbury officials replied yes, Parker's order, bearing the name of the already-freed Ignacio Novo, apparently was sent, Barcella said.

The presence of Ignacio's name on the order confused Danbury officials, who tried unsuccessfully to reach someone in the court clerk's office to clear up the name problem. Barcella said his understanding was that they then called the U.S. Marshal's Office and officials there simply confirmed that a "Novo Sampol" was to be released. Shortly thereafter, Guillermo Novo walked out of Danbury, Barcella said.

Guillermo Novo and a codefendant, Alvin Ross Diaz, were acquitted of murder and conspiracy charges last spring in connection with the killing of Letelier and a young associate in September 1976. They were convicted of the same charges in 1979 but had won a retrial from the U.S. Court of Appeals.