As a literary whodunit, it would make good reading on its own. But this real-life murder mystery has excited the imagination and curiosity of Spaniards and the wider world for 70 years.

Shrouded in secrecy for decades, the answer to the riddle of who killed the country's most celebrated poet and playwright, Federico Garcia Lorca, has remained elusive. But on the eve of the 70th anniversary yesterday of the poet's execution by firing squad, a new film has been shown which reveals for the first time who were his murderers.

The documentary makers claim jealous members of his own middle-class family in Granada, southern Spain, helped to organise the death squads which ended the life of Garcia Lorca, a republican sympathiser.

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They claim Horacio and Miguel Roldan, cousins of Garcia Lorca, were behind the killing.

Part of the documentary, Lorca, el mar deja de moverse (Lorca, the calmed sea) was shown at a remembrance service near Granada last night.

Within weeks of the start of the Spanish Civil War, death squads sympathetic to General Francisco Franco's uprising carried out indiscriminate shootings of hundreds of republicans across southern Spain.

Garcia Lorca was denounced as a republican, a Communist and a homosexual. Two days after being arrested, he was shot and his body dumped in a mass grave, whose location remained unknown for years.

According to the film, which will be shown in full next month, the murder was partly the result of a family feud. Emilio Ruiz Barrachina, the documentary maker, says new evidence suggests Garcia Lorca's death was the result of simmering family rivalries.

Parts of the family were outraged by the huge success of his play The House of Bernarda Alba, about an oppressive Andalusian matriarch, and believed it was a personal slight. The political sympathies of Garcia Lorca's family for the republican government were also at odds with those of the Roldans, who backed Franco.

The poet's open homosexuality helped to seal his fate. The man who is alleged by the programme makers to have shot Garcia Lorca, is named as Juan Luis Trescastro Medina. He was said to have boasted he murdered Garcia Lorca because he was gay. On the night of the murder, Trescastro was said to have shouted in a bar: "I put two bullets in his arse for being a homo!"

Miguel Francisco Caballero, the author of the research for the film, said Trescastro was a distant relative who is buried in the family tomb.

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Despite the rapid advance of troops led by Franco, after his uprising in July 1936, Garcia Lorca travelled from Madrid to be with his family in the south.

Even as Franco's troops overran Granada, Garcia Lorca refused to try to cross back towards republican lines. The poet was attacked and humiliated by pro-Franco troops but escaped with his life and sought the help of influential friends to escape. Hiding at a friend's house in Granada, he learnt of the execution of his brother-in-law, Jose Fernandez Montesinos, who was a former Socialist mayor. The same day he was arrested and taken to a prison in the town.

Angelina Cordobilla, who worked for the Garcia Lorca family, took food to the jail a couple of days later only to be told the poet was not there any more: he had been taken the night before to be shot by firing squad nearby.

The death of Garcia Lorca, whose other celebrated works include the play Blood Wedding and his book of poems Poet in New York, has been a controversial subject in Spain for decades.

His political sympathies immediately made him a martyr for the the republicans who were defeated during the Civil War.

Afterwards, during the Franco dictatorship, his works were banned. Since the death of El Caudillo in 1975, there have been a number of attempts to discover who really did kill Spain's greatest playwright and where he was buried. These have been hampered to a great deal by the stubborn refusal of Garcia Lorca's family to co-operate in efforts to discover finally what became of their famous relative.

Ian Gibson, the Irish-Spanish historian and biographer of Garcia Lorca, tracked down his remains to a mass grave under an olive grove near Granada. But the family have refused to allow the grave to be opened.

His niece Laura Garcia Lorca has said: "We are not going to discover new facts whose importance justifies the violence of disinterring the dead."

But the relatives of two anarchist bullfighters and a teacher whose bodies lie in the same mass grave with Garcia Lorca, want to identify their remains. And many believe discovering what really happened to a literary figure of the stature of Garcia Lorca would focus attention on the 30,000 other victims of Franco's death squads who are thought to lie in up 800 mass graves across Spain.

Mr Gibson said yesterday: "They often say that a country which doesn't remember its past will be forced to repeat it. I would stress the young and the old should learn what really happened [to Garcia Lorca] so it never happens again." Perhaps as a sign that they might be relenting, the poet's family did help the film-makers, providing new material from local archives.

The Spanish government last month introduced a new law which will honour all of Franco's victims. Part of the legislation involves producing a map of the country which details the final resting places of those whose bodies were dumped in mass graves during the Civil War and in the repression afterwards.

This morbid map will allow relatives to finally pay their respects to fathers, grandfathers and mothers, whose graves have been unknown for years.

The Popular Party, Spain's conservative opposition, has claimed that the new law will only ignite the old quarrels which split Spain until it underwent its transition to democracy after Franco's death.

Lorca's life

* Born in 1898, in Fuente Vaqueros, to a farming family.

* Studies law and philosophy at the universities of Granada and Madrid.

* Emerges from university a published writer. He is friends with Dali, Neruda, Jimenez and Bunuel, and is a leading member of the "Generation of '27".

* Best known for the poemLament for the Death of a Bullfighter and three Andalusian plays, Blood Wedding, Yerma and The House of Bernarda Alba.

* His work sympathises with the provincial poor and the ideals of the new Spanish democracy. His theatre group, La Barraca, takes his work to rural audiences.

* Supports republican Popular Front in the Civil War.

* Arrested by nationalists in July 1936 with his brother-in-law, the socialist Mayor of Granada. Shot on 19 August.