The names of five black and Hispanic employees were attached to photos of people hanging with ropes around their necks, The Palm Beach Post reported Wednesday.
City officials say they think the envelope titled "Happy Halloween" was placed on the truck, parked in a city maintenance lot, by an employee on Oct. 31.
The label maker used to affix the names to the pictures is the kind used in the city's utilities department, the newspaper said.
"This was an unacceptable and deplorable act," Mayor Jeri Muoio said in a statement. "The city has investigated the incident and will continue to work to try and identify the responsible party. In the event the responsible party was a city employee, they will immediately be terminated."
One of the targeted employees criticized the city for not being more aggressive in pursuing the incident.
"Me and everybody else was puzzled, and everyone is still really mad about this," the employee told the newspaper. "Human resources, I thought they'd step up, but they didn't do enough. If they worked harder, I believe they could have done a lot better than they did."