Federal investigators have gathered evidence leading them to believe that the murder of former Chilean Ambassador Orlando Letelier here last September was carried out by anti-Castro Cubans directed by persons in Chile, according to sources familiar with the investigation.

The new focus of the Letelier probe was discussed at a recent FBI headquarters meeting attended by Justice officials and FBI agents involved in the case, the sources said.

Within the last two weeks, according to sources, at least six members of a Miami-based group of anti-Castro Cuban veterans of the Bay of Pigs invasion, known as Brigade 2506, have been called as witnesses before a federal grand jury here.

These new developments, which some investigators cite as "significant progress" in the investigation of the bombing death of letelier, stem from detailed information the investigators have obtained from "sensitive sources" with the cooperation of the Central Intelligence Agency, according to the sources.

These "sensitive sources" include informants in the U.S. and abroad, according to investigators. "Sensitive sources" also is a term used in the intelligence community to identify information obtained from electronic surveillance, but no investigator would describe the "sensitive sources" in this case further.

Assistant U.S. Attorney Eugene M. Propper, who is coordinating the federal investigation, said he could not respond to a reporter's questions about the case. He said "progress" was being made, but could not provide details.

Letelier, 44, former ambassador to the United States, foreign minister and minister of defense in the Marxist government of the late Chilean President Salvador Allende, was killed Sept. 21 when a bomb exploded beneath his car as he drove along Embassy Row here.

One passenger in the car, Ronni K. Moffitt, a staff member of the Institute for Policy Studies, also died in the explosion. her husband, Michael, a research associate at the institute, survived the blast. At the time of his death, Letelier headed a foreign affairs research program at the institute, a leftist think tank.

Among those called to testify before the grand jury investigating letelier's death, according to informed sources, was Roberto Caballo, who is president of Brigade 2506. Caballo could not be reached yesterday for comment.

Members of Brigade 2506 took part in the abortive invasion at the Bay of Pigs in 1961. The brigade has existed since then, according to federal authorities in Miami and others familiar with it, as a loosely organized association of veterans of the invasion. One member said yesterday that it now includes 800 to 900 members.

Brigade 2506 has been reported to have taken part in formation a right-wing, anti-Castro umbrella organization, known as CORU, the coordination of United Revolutionary Organizations. The umbrella group is headed by Orlando Bosch, who has been detained by Cenezuelan authorities in connection with the Oct. 6 crash of a Cuban airliner, in which 73 persons died. The plane is believed to have been blown up.

Caballo later disavowed support for CORU.

One Brigade 2506 veteran, Rolando Otero, was convicted in circuit court in Florida last Thursday of a 1975 bombing incident at Miami International Airport, which caused considerable damage, but not injuries.

Juan Jose Peruyero, a former president of Brigade 2506 who was viewed as a political moderate among anti-Castro Cubans, was fatally shot Jan. 7 outside his Miami home. His assailants have not been arrested.

(TEXT OMITTED FROM SOURCE) have not yet received any response wanted for questioning in the Letelier investigation. Federal prosecutors have, however, been thwarted in attempts to interview him in Caracas, where he has been detained.

Federal investigators originally had planned to go to Veneuela in November to interview Bosch, but then learned they would not be allowed to interview him because of a Venezuelan law limiting foreign interrogations of persons detained there.

In December, the federal investigators sent a list of questions to a Venezuelan court with the request that they be given to Bosch. The investigators also formally asked again that person.

State Department officials say they have not yet received any respons from the Venezuelan government. According to news service reports from Caracas, Bosch has refused to answer the questions because of his right against self-incrimination.

The detention of Bosch and others held in connection with the Cuban airliner crash appears to have posed a touchy political problem for Venezuela. An acquittal of the suspects in the Cuban plane crash, U.S. analysts say, could endanger Venezuela's warm relations with Prime Minister Fidel Castro's government in Cuba and might anger the Venezuelan left. If Bosch and the other suspects are convicted, the verdict, State Department analysts say, could prompt protest among rightist Cubans living in Venezuela.

Both political and financial motives may have led anti-Castro Cuban exiles to have taken part in Letelier's killing, according to informed sources.

Many anti-Castro Cubans now believe their cause has been abandoned by the U.S. government and regard the current Chilean military regime as a South American bastionof anti-Communism.