GUATEMALA CITY, July 14 (Reuters) - Gunmen shot dead on Monday a Guatemalan state prosecutor who was investigating the murder last year of three Salvadoran deputies from the Central American parliament, police said.

Juan Carlos Martinez, part of a government team probing the murders, was shot while driving his car near his home south east of Guatemala City, said Faustino Sanchez, a spokesman for the national police.

"The prosecutor was attacked and shot several times and his body was slumped in the car he was driving," Sanchez said.

The charred bodies of the representatives to the Central American parliament were found riddled with bullets on a back road in February 2007.

The parliament is a political body uniting six countries which promotes regional integration.

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

Martinez was also investigating the April murder of Victor Rivera, a former security advisor to the interior ministry.

Rivera was involved in the investigation of the murdered congressmen, but rights groups say he was also operating police death squads from his government post.

Authorities say drug gangs with links to Guatemala and El Salvador were behind the killings, which have shed light on illegal armed groups operating within Guatemala's security forces and possible links between high-level officials and narcotics traffickers.

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.

"Each year there are more than 100 cases of violence against justice officials and lawyers, and there is strong evidence that the problem has gotten worse in the past six months," said human rights group the Myrna Mack Foundation in a statement.

"There is an atmosphere of terror in the justice system, particularly in the most high-profile cases," it said.

(Reporting by Herbert Hernandez, editing by Alan Elsner)

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