WASHINGTON (AP) _ Cuban military forces in 1980 sank an excursion boat being hijacked to the United States, killing many of the passengers aboard, according to Radio Marti, the United States broadcast service to Cuba.

Ernesto Betancourt, director of Radio Marti, said in a telephone interview on Sunday that after an investigation, including interviews with Cuban exiles, the station produced a 36-minute report on the sinking and broadcast it to Cuba.

The twice-a-day broadcasts began Thursday and ended Sunday, Betancourt said.

Betancourt said that on July 6, 1980, at a time when thousands of Cubans were fleeing to the United States, two military draftees commandeered a cruise boat with 70 people aboard on the Canimar River near the seaport of Matanzas.

As the craft headed out of the harbor heading north, a patrol boat and an airplane opened fire.

The boat then sank, either by the shooting or after it was rammed by a dredge, Betancourt said.

He said none of the informants interviewed by Radio Marti, including five who spoke on the broadcast, saw the sinking, but had talked with survivors and had seen the turmoil in Matanzas as a result of the disaster.

''Most governments try to settle hijackings without endangering the lives of innocent people,'' Betancourt said. ''This is not what happened in this case.''

''We know that the government threatened survivors and their families by telling them they would go to prison if they talked about what happened,'' Betancourt said. ''Officers went to funeral homes to make sure relatives didn't say anything.''

He said the station did not reveal the names of the Cuban exiles who were heard on the broadcast, to prevent retaliation against relatives back home.

Betancourt said the station first heard of the incident from exiles and spent many weeks verifying the report because it wanted to make sure all the details were accurate before making the broadcast.

He said the delegation representing Cuban interests in the United States had no response to the report, in keeping with a policy of declining to comment on Radio Marti or its broadcasts.

Radio Marti, a subsidiary of the U.S. Information Agency, went into operation on May 20, with the mission of beaming news, entertainment and service programs to Cuba, whose airwaves are dominated by government-sanctio ned stations.

Betancourt said one of the goals of Radio Marti is to give Cubans news of their own country, but that little of this has been done because of the difficulty of getting timely information from the island.

The report on the sinking of the cruise ship was the first major effort to provide such information, Betancourt said.