The leak is hugely sensitive in Northern Ireland, where officers are still sporadically targeted by dissident groups in bomb and gun attacks, despite a 1998 peace deal largely ending three decades of sectarian violence.
"We continue to work toward establishing those who possess information relating to last week's data breach," Detective Chief Superintendent Andy Hill said in a statement.
The surnames, initials, work location and departments of all serving Northern Irish police officers and staff, numbering more than 9,000, were available to the public online for more than two hours last week, after they were included in error in response to a freedom of information request by a member of the public.
A 39-year-old man was arrested as part of the investigation on Wednesday.
(Reporting by Graham Fahy; Editing by Nick Macfie)
