SAN SALVADOR, June 18 (Reuters) - Salvadoran President Tony Saca complained on Monday about neighboring Guatemala's lack of progress in solving the murders of three Salvadoran politicians in Guatemala in February.

The bodies of the three lawmakers and their driver were found bullet-ridden and charred beyond recognition on a dirt track an hour's drive east of Guatemala city.

Days later, four police detectives arrested in connection with the murders were shot in prison.

Four months on, Guatemala has arrested a police officer and five other people linked to a drug cartel based near the border with El Salvador and is investigating them, but detectives have not established a motive for the murders.

Saca said he would speak to Guatemalan President Oscar Berger about the issue when they meet at a June 28-29 regional integration summit in Belize.

"I expect to pass on to him the concern of the Salvadoran people for this lack of progress. I think they should be getting more done," he said. "We expect more of Guatemala, up to getting to the bottom of these killings."

Eduardo D'Aubuisson, William Pichinte and Jose Ramon Gonzalez were deputies in the Guatemala-based regional Central American parliament from El Salvador's conservative ruling ARENA party. They went missing during an overland trip to Guatemala from neighboring El Salvador.

D'Aubuisson was the son of the late Roberto D'Aubuisson, an infamous 1980s death squad leader during El Salvador's civil war who went on to found ARENA.

Medical examiners said each deputy had a single bullet wound in the skull and the bodies were so badly burnt that dental records were needed to identify them.

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