Ozaukee County has been hacked.
"Our employee portal may have been compromised in a sophisticated data breach," said County Administrator Thomas Meaux.
County workers have become victims of identity fraud.
The employees were not aware of the theft until they filed their 2015 taxes and received a fraud notice from the IRS.
"About 20 employees (so far)" have had their identities stolen, according to Meaux.
After investigating the portal, the county believes information from some 200 workers may have been compromised.
The Ozaukee County Sheriff's Office is now investigating.
Workers who may have been hacked are asked to contact the Sheriff's Office.