Guatemala's military has been forced to open up its records to officials investigating alleged corruption under former President Alfonso Portillo.
Portillo left Guatemala under a cloud
Investigators are trying to trace about $100m in funds allegedly transferred to the military between 2001 and 2003.
The army argued that its records were a state secret - an argument rejected by an appeals court on Friday.
Mr Portillo left Guatemala for Mexico in February under a cloud of corruption allegations - which he denies.
The former president has said he fled because he could not get a fair trial in Guatemala.
President Oscar Berger - who succeeded Mr Portillo in January - welcomed the investigation, saying it was part of the transformation of the military.
Three army generals have been banned from leaving Guatemala while the inquiry proceeds.