Koster said in a release Monday that the lawsuit also named City Administrator Henry Luxem, Mayor Constant Craig, and Joseph Brauer, chief of police.
The lawsuit accuses Brauer of sending an email in December 2010 to police department employees informing them that the city council had reversed a police board decision to discipline an employee.
Koster said the city then denied an open records request seeking the email and related documents.
Koster's office sent Luxem a letter stating that emails sent on, received by, and retained on the city's computers are public records that must be disclosed under the Sunshine Law.
City officials did not immediately return calls seeking comment.