Four Guatemalan policemen have been arrested in connection with the murders of three Salvadorean politicians and their driver near Guatemala City.
The three men were deputies to the Central American Parliament
The arrests follow a request by El Salvador and Guatemala for the FBI to help with the inquiry into the attack.
The men's bodies were found late on Monday in their burnt and bullet-riddled vehicle.
Forensic experts said at least two of the victims had been alive when they were set on fire.
The three politicians - Eduardo D'Abuisson, William Pichinte and Ramon Gonzalez - were members of El Salvador's governing Arena party and were deputies to the Central American Parliament.
One was the son of Arena's late founder, Roberto D'Aubuisson, who was accused of being behind the assassination of Archbishop Oscar Arnulfo Romero in 1980.
There has been speculation that the murders were politically motivated.
El Salvador's Attorney General Felix Garrid Safie said the killings were almost certainly linked to organised crime.
Police had launched a search for the four men late on Monday afternoon after they received a report that they had not yet arrived at their hotel.