City manager Dave Hansen said the victims - 11 local authority workers and one contractor - "leave a void we will never be able to fill".
The gunman shot indiscriminately before dying in a gun battle with police.
He has been identified as DeWayne Craddock and described by police as a disgruntled city employee.
Authorities said they would only refer to him by name once, in order to focus attention on the victims.
