SAN SALVADOR, July 15 (Reuters) - El Salvador's President Tony Saca blamed drug gangs on Tuesday for the killing of a Guatemalan state prosecutor who was investigating the murder of three Salvadoran deputies to the Central American parliament.

"We are talking about the big league, powerful drug cartels that are doing everything possible to keep people from knowing the truth," Saca told a news conference.

Juan Carlos Martinez, part of a Guatemalan government team probing the murders, was shot on Monday while driving his car near his home southeast of Guatemala City.

The charred bodies of the three representatives to the regional parliament and their driver were found riddled with bullets on a back road in Guatemala in February 2007.

The lawmakers were members of El Salvador's ruling right-wing Arena party, which has held office since 1989.

Four policemen traced to the crime scene by a global positioning device in their car were arrested but killed days later in a maximum-security prison cell before they could testify.

"Bigger things are going on here and someone is blocking us from knowing what they are," said Saca, a staunch U.S. ally.

Guatemala has one of the highest murder rates in Latin America and has become a major hub for drug traffickers moving cocaine from Colombia through Mexico to the United States.

A failing justice system with high levels of corruption means few crimes are solved and lawyers and investigators are increasingly attacked, rights groups say. (Reporting by Alberto Barrera, Editing by Anthony Boadle)

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