The man, dressed as a prison chaplain, told family members he needed to do a report on former People's Party treasurer Luis Barcenas, who has been in custody since June on corruption charges, The Local.es reported Thursday.
The suspect, identified only as "Olivares," pulled a gun and demanded the family give him "a [flash] drive to oust the government."
He tied them up and held them hostage for an hour before Barcenas' son Guillermo broke free and restrained him. A neighbor heard the mother's cry for help and called police.
Olivares reportedly had psychological problems and a criminal record.
The computer device he was after allegedly contained records of illicit payments made by Barcenas to People's Party members.
Police said Olivares' gun contained only blanks. They did not indicate if he was in custody.