Four people have been arrested in connection with the killing of three Salvadoran politicians and their driver near Guatemala City
The alleged members of a drug cartel are believed to be behind the kidnap and killings allegedly carried out by corrupt police officers.
Officials previously identified six police as suspects over the killings.
Last month, four policemen were arrested over the attack and were subsequently killed in prison.
A fifth police officer, now in detention, is being kept under close police protection. The sixth suspect is still at large.
The bodies of the three Salvadorian politicians were found in a burnt and bullet-ridden vehicle on 19 February.
They had just arrived in Guatemala when they were kidnapped. Two of them were burned alive, and the driver and the third politician were killed before their bodies were set afire, a post-mortem showed.
The four new suspects - three men and one woman - helped police search the politicians' car for drugs and bought petrol used to torch the vehicle and burn the bodies, Interior Minister Carlos Vielmann said.
"They were the ones that took apart the lawmakers' car looking for money or drugs, and we know that they are among the masterminds of the killings."
The arrests were made after raids on houses in the province of Jutiapa, close to the border with El Salvador.
The latest detentions come a day after the Salvadorian president, Tony Saca, urged the Guatemalan authorities to do more to find the culprits.
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