Federal investigators now believe the bombing murder of former Chilean ambassador Orlando Letelier was carried out by anti-Castro Cuban exiles at the direction of Chile's secret police unit known as DINA, The Washington Post has learned.

Letelier and a fellow employee at the Institute for Policy Studies, Ronni K. Moffitt, were both killed last Sept. 21 on Sheridan Circle NW by a bomb hidden under the car Letelier was driving. The murders have prompted an unusually extensive FBI investigation into the shadowy world of foreign intrigue that surrounded the unprecedented political assassination of a former foreign official on a Washington Street.

The Washington Post reported last February that certain anti-Castro Cuban exiles and Bay of Pigs veterans in a group known as Brigade 2506 were the major suspects in the bombing and that they may have served as assassins-for-hire at the request of then-unnamed "persons in Chile."

Now, however, investigators have determined to their satisfaction that the bombing was either directed by officials of DINA or persons acting on behalf of DINA officials, Justice Department sources said yesterday.

The Justice Department sources refused to explain why they now believe DINA was involved.

Letelier, 44, was the former ambassador to the United States, foreign minister and minister of defense in the Marxist government of the late Chilean President Salvador Allende.

After the Allende government was overthrown in September, 1973, the military junta now headed by Augusto Pinochet, he was imprisoned for one year.

An economist who had worked for the Inter-American Development Bank in Washington for a decade before becoming Allende's ambassador, Letelier had written and spoken widely in opposition to the present Chilean government. Its critics, including Letelier, denounced the country for alleged torture, political repression and imprisonment of its opponents.

Before its dissolution two weeks ago, Chile's Directorate of National Intelligence (DINA) had been the center of the controversy about human rights violations in Chile because of its powers to make secret arrests without charges and maintain hidden interrogation centers where political prisoners are held.

International human rights organizations had accused the feared Chilean secret police of being responsible for the torture and disappearance of hundreds of political dissenters.

Columnists Jack Anderson ands Les Whitten report in a column in today's Washington Post that investigators are able to trace the order to murder Letelier directly to DINA chief Manuel Contreras Sepulveda. However, one knowledgeable Justice Department official discounted that report as "totally incorrect" while at the same time conceding that Contreras probably would have known of any such acts carried out at DINA's direction.

The accusation that DINA is involved in the murder of Letelier could have wide-ranging effects on diplomatic relations between the U.S. and Chile, whose president, Pinochet, was in Washington for the signing of the Panama Canal treaty yesterday.

State Department officials could not be reached for ocmment on the allegations concerning DINA late yesterday, and Justice Department officials refused to discuss any negotiations that may be in progress between State and Justice concerning the probe.

Assistant U.S. Attorney Eugene M. Propper of the major crimes division, who has been coordinating the Letelier investigation for the past year, confirmed only that he has consistently "maintained contact" with the State Department about the case. He said he would not otherwise comment on the case.

Since the early-morning murders last September, the investigation by a federal grand jury here and the FBI here and abroad has been conducted in unusually tight secrecy. It is known that the Central Intelligence Agency and intelligence agencies from foreign countries also have been cooperating with the Justice Department probe.

For the first few months of the investigation, there were no significant leads in the case as probers sifted through numerous theories and possible suspects. However, by late January it became clear that anti-Castro Cuban exiles who were opposed to Letelier's highly visible Socialist stand had become the focus of the probe.

One Cuban exile now living in Elizabeth, N.J., has been in jail for five months rather than testify before the grand jury after being granted immunity from prosecution.

Jose Dionisio Suarez, 38, said at the time he was refusing to testify because the Letelier investigation was being used to harass the Cuban exile community in the United States. He is a member of the CUban Nationalist Movement, a right-wing group that operates out of the New Jersey area.

Guillermo Novo, another member of that Cuban group, has been a fugitive since June 7 after he failed to appeear at a Trenton, N.J., hearing on whether his parole on a 1974 explosives conviction should be revoked.