Officials in Athens on Tuesday that police arrested a man on suspicion of stealing personal data for almost two thirds of the country's population.
Police said the man - a computer programmer 35-year-old - suspected also tried to sell 9 million files containing personal data, addresses and card numbers and tax figures plates for cars.
The number of Greece's population of 11 million.
And spokesman Christos Manoras said police spokesman told reporters "We are investigating to find out the source of the data used and how the man who was arrested as well as the possibility that it provided to another person."
Officials said the police trying to find out whether the man had obtained the data files by penetrating government Internet server and whether his partner. Police discovered the files after a raid on the man's house.
The man did not go to any charges so far, but is expected to appear before a prosecutor later.