BUENOS AIRES (Reuters) - An Argentine judge has started an investigation into the death of Spanish poet and playwright Federico Garcia Lorca, who is believed to have been executed in 1936 by forces loyal to General Francisco Franco.

Garcia Lorca's fate remains a mystery after the site near the Spanish city of Granada where he was believed to have been buried was excavated in 2009 without finding human remains.

With efforts by the Spanish justice system stalled, a Spanish human rights group called the Association for the Recuperation of Historic Memory asked Argentine federal judge Maria Servini to take up the case. She has accepted, said a statement by the group on Facebook.

"The case has been incorporated into an ongoing investigation by Judge Maria Servini into crimes against humanity," it said.

Servini was already looking into Franco-era crimes ranging from torture to extra-judicial killings. Franco ruled for almost four decades after his Nationalist forces won Spain's 1936-1939 civil war.

Spain's most famous human rights judge, Baltasar Garzon,

opened an inquiry into Franco-era crimes in 2008 but later dropped the case - an example of the issue's political sensitivity.

The Spanish group first requested that Servini, who could not be reached for comment, take up the Garcia Lorca case in April.

"The judge has requested that the courts in Madrid release the case file to the association," an Argentine court source with knowledge of the case told Reuters. Unauthorized to speak to the press, the source asked not to be identified.

Spain's civil war became a curtain-raiser for World War Two when Nazi Germany and Fascist Italy provided arms and funding for Franco's forces. Soviet leader Josef Stalin backed communists fighting against them and Franco saw himself as a "sentinel" against communism.

Volunteers from various countries, known as the International Brigades, traveled to Spain to join the fight against Franco after he launched his revolt against Spain's Republican government in 1936.

Historians estimate as many as 500,000 combatants and civilians were killed on the Republican and Nationalist sides in the war. After it ended, tens of thousands of Franco's enemies were killed or imprisoned in a campaign to wipe out dissent.

(Reporting by Maximilian Heath; Writing by Hugh Bronstein; Editing by Peter Cooney)