Detectives from Serious Crime Branch investigating the theft of documents from PSNI arrested a 62-year-old man this week.
The suspect was arrested after officers conducted a search of a property in the Hillsborough area earlier on Wednesday.
Another search was conducted in the south of England. A number of items have been removed for further examination.
The 62-year-old man is being questioned at Lisburn police station.
Police do not believe the security of any individual has been compromised.