The politicians from El Salvador were killed in February last year
Police in Guatemala say gunmen have shot dead a state prosecutor who was investigating the murders of three Salvadorean politicians last year.
Juan Carlos Martinez was shot while driving near his home in a suburb of Guatemala City, police said.
The authorities say drug gangs were behind the killing of the Salvadorean politicians, who were members of the Central American Parliament.
Guatemala has one of the highest murder rates in Latin America.
It has become a major hub for drug traffickers moving cocaine from Colombia through Mexico to the US.
The bodies of the three Salvadorean politicians were found in a burnt and bullet-ridden vehicle in February 2007.
The alleged members of a drug cartel are believed to have been behind their kidnap and killings.
Four policemen who were arrested in connection with the murders were subsequently killed in prison before they could testify in court.
Last week, a Guatemalan court acquitted 13 alleged gang members of the police officers' murders, saying there was a lack of evidence.
Prosecutors had said they would appeal against the court's decision.
Mr Martinez was also examining the murder in April of Victor Rivera, a former security advisor to the interior ministry who was involved in the investigation, Reuters news agency reports.